It's every bosses goal to get the very best out of their EA. And although EAs try to improve themselves through various channels, there is a certain approach all bosses should take with their EAs to ensure they are getting the best from them. Now I am, and always will be, a massive believer that when it comes to the work style that exists between an Executive and their EA its not a two way street. In fact its up to the EA to adapt to the Executive's style of work, not the other way around. But there are some common courtesies that should be applied from both sides of the relationship to ensure the relationship is propped up by some essential pillars, namely loyalty and trust. To me the below rules of engagement that I've detailed are purely logical. But sadly where they are logical to some, they are not always logical to others, namely those Executives that struggle with emotional intelligence (and we've all worked for at least one of them!). But whether you've got a fantastic boss that does all of the below, or not, its always good to pause and review your current working relationship and evaluate where there is room for improvement. If you find you working relationship is lacking any of the below, it might be time for some action, or at the very least time for a bit of a shake up. You might consider discussing the below with them in your own subtle way, as if they want to get the very best from their assistant, which all they all do, they will want to listen to what you've got to say very closely.
1. Trust: Trust should always be given, not earned. There are so many 'trusts' required from an Exec and their EA that it really should all just be handed over in full. Relationships suffer where there is no trust, even in the beginning. When there are limitations and hesitations around tasks that are delegated, then ultimately the tasks cannot be carried out properly or efficiently. Essentially there are many different types of trust that exist in a working relationship, for example: the Executive should trust that particular jobs and tasks will get done by the EA; they should trust that the EA will do what they say they will do; and they should trust that they will protect them at all costs - from their reputation to the protection of their personal information. On the other side of the coin however, the EA doesn't have as many requirements of trust, just one - and that is that they trust that their boss will return the favour, protect them and have their back in times of need (i.e at times of redundancy, company restructure, or pay reviews etc). A difficult bit of trust to hand over, especially when it needs to be seen to be believed, but if they are trusting you without hesitation than the courtesy should be returned. But if the trust is broken, then you are well within your rights, as they are, to return to cautiousness.
2. Loyalty: A build on the last comment, loyalty again is a two way street. Where you give your loyalty to them, they in turn should give their loyalty to you. EAs do their utmost to simplify and ease their bosses lives, ensure they are protected and guarded from anything untoward and basically deliver everything that they're asked of. In turn, the loyalty should be returned. Bosses should protect their assistant in every form, stand up for them in their time of need and be their biggest advocate. Essentially they should have your back. You are the closest person to them, outside of their husband/wife, and that should speak volumes. So if you find yourself in a work relationship with an Exec who isn't putting you first and prioritizing your needs/requests then there is something wrong and you should address it.
3. Prioritize: Similarly, you should always be your bosses priority, especially when it comes to responding to emails/calls/texts. Your messages should always be responded to, and where possible first, especially when you're urgently hunting them down. It is just downright unacceptable to be ignored, not only is it frustrating for the EA, but it is just plain rude. We all know our Exec's are extremely busy and cannot always answer their phone or respond to your messages when they are in meetings, but when urgency calls they should be responsive to you, if to no one else. Sometimes its necessary to set up a 'code' for your emails/messages that need an urgent response, mark the subject or start the messages with 'URGENT' and make them aware that those particular messages will require an immediate response. Additionally, they should make time to meet with you at least once a week to review the diary and do some essential planning. As clever as EAs can be, we are not mind-reader's and sometimes do actually need some guidance.
4. Boundaries: The boundaries should be established from the get go, but are also something that will develop and be discovered as time progresses. Its essential to know what responsibilities are yours and what are theirs. And where responsibilities have been delegated to the assistant, the trust must be there to know the task will in fact get carried out. As an EA there is nothing worse than being micro-managed on tasks that have been delegated to you to manage. The lines should not be blurred on tasks/responsibilities and boundaries should not be crossed. For when this happens it only puts elements of doubt in the assistant's mind as to whether they are sincerely trusted, or worse still, capable of doing the task that has been delegated to them at all.
5. Don't ask twice: Ever been asked to do something once, only to have a little time pass, and then get asked again, and again?? Yes it drives us nuts, and its something that just shouldn't happen. When an assistant is asked to do something there needs to be an element of trust that it will get done. Similarly its as frustrating when an assistant knows a certain task is his/hers, yet the Executive asks them to do it anyway (and even more frustrating when its already been done!). Its like the theory of being innocent until proven guilty, if the assistant knows a certain job is to be done there must be trust that it will happen. Questions should only be asked when it appears the task has not been done. Its necessary to always give the benefit of the doubt.
6. Delegating: Its the bosses role to ensure the tasks delegated to the EA don't stray too much away from the essence of what is collectively trying to be achieved. EAs shouldn't get bogged down too much on project work or dragged in to tasks that sit outside of the realm of their role. Every project they are looped in to, and every job they get delegated ends up being just another task that takes away from their priority - the boss! And when EAs get spread to thin the quality of their work will suffer as a result. Its the Executive's job to protect them from being dragged into unnecessary project work. Without micromanaging, the Executive should always be aware and conscious of the EAs workload, and be conscious that they are not overloaded or straying to far from the essence of what they are both trying to achieve. The EA is essentially the 2IC to the Executive, if the EA is off working on projects that have little or nothing to do with the Executive's priorities then the relationship is clearly faltering. Both the EA and the Executive should always be working on the priorities at hand together. The EA is there to help manage the workload of the Executive, not to get pulled in on every project because of their administration and organisational skills.
7. Growth: There is nothing better than when your boss genuinely cares about your career and your future growth. Even if the assistant is amazing and one not to be lost, they should still be encouraged to grow and develop in the areas that they would like to (even if this means moving on). EAs should always be encouraged to take courses and attend master classes, as at the end of the day, a strengthened skill set will only benefit the Executive in the short term anyway.
8. Value: Its essential to remember that EAs are experts in management. No longer are we considered as just the 'secretary' with a skill set of just typing and dictation. The role of the EA has developed beyond that. Although sometimes I think sadly our bosses can forget all the balls we juggle at one time, and rarely ever dropping one (that is unless they start throwing more unnecessary balls in to our act). From seamless diary management, to arranging complicated travel arrangements and agendas, to running meetings, managing approvals and processes, leading administrative teams and event management, we seriously do it all. And we should always be valued for the multi-skilled abilities we have. Our opinions matter, our opinions should be trusted and they should always be sought after. Although the Exec might be the 'big boss', it doesn't always mean that they have the best lateral approach to administrative tasks. With all the experience we hold, it would be crazy not to engage us and seek our opinion on certain things. We are at the end of the day, the closest person to the boss, and that is something that everyone, including the boss, should remember.
Its difficult to find those amazing bosses, the ones that will always have your best interests at heart, and that will be as loyal to you as you are to them. My advice is that if you find one, then hang on to them! And that is no doubt why many EAs follow their Exec's from company to company or job to job. But not all Execs and their EAs are a match made in heaven, often the relationships need time to grow. Good things do take time, and where it is good to have trust and loyalty handed to you from the start, its something that never the less, that will just get stronger and stronger if proved over time.
3. Prioritize: Similarly, you should always be your bosses priority, especially when it comes to responding to emails/calls/texts. Your messages should always be responded to, and where possible first, especially when you're urgently hunting them down. It is just downright unacceptable to be ignored, not only is it frustrating for the EA, but it is just plain rude. We all know our Exec's are extremely busy and cannot always answer their phone or respond to your messages when they are in meetings, but when urgency calls they should be responsive to you, if to no one else. Sometimes its necessary to set up a 'code' for your emails/messages that need an urgent response, mark the subject or start the messages with 'URGENT' and make them aware that those particular messages will require an immediate response. Additionally, they should make time to meet with you at least once a week to review the diary and do some essential planning. As clever as EAs can be, we are not mind-reader's and sometimes do actually need some guidance. 
4. Boundaries: The boundaries should be established from the get go, but are also something that will develop and be discovered as time progresses. Its essential to know what responsibilities are yours and what are theirs. And where responsibilities have been delegated to the assistant, the trust must be there to know the task will in fact get carried out. As an EA there is nothing worse than being micro-managed on tasks that have been delegated to you to manage. The lines should not be blurred on tasks/responsibilities and boundaries should not be crossed. For when this happens it only puts elements of doubt in the assistant's mind as to whether they are sincerely trusted, or worse still, capable of doing the task that has been delegated to them at all. 
6. Delegating: Its the bosses role to ensure the tasks delegated to the EA don't stray too much away from the essence of what is collectively trying to be achieved. EAs shouldn't get bogged down too much on project work or dragged in to tasks that sit outside of the realm of their role. Every project they are looped in to, and every job they get delegated ends up being just another task that takes away from their priority - the boss! And when EAs get spread to thin the quality of their work will suffer as a result. Its the Executive's job to protect them from being dragged into unnecessary project work. Without micromanaging, the Executive should always be aware and conscious of the EAs workload, and be conscious that they are not overloaded or straying to far from the essence of what they are both trying to achieve. The EA is essentially the 2IC to the Executive, if the EA is off working on projects that have little or nothing to do with the Executive's priorities then the relationship is clearly faltering. Both the EA and the Executive should always be working on the priorities at hand together. The EA is there to help manage the workload of the Executive, not to get pulled in on every project because of their administration and organisational skills.
8. Value: Its essential to remember that EAs are experts in management. No longer are we considered as just the 'secretary' with a skill set of just typing and dictation. The role of the EA has developed beyond that. Although sometimes I think sadly our bosses can forget all the balls we juggle at one time, and rarely ever dropping one (that is unless they start throwing more unnecessary balls in to our act). From seamless diary management, to arranging complicated travel arrangements and agendas, to running meetings, managing approvals and processes, leading administrative teams and event management, we seriously do it all. And we should always be valued for the multi-skilled abilities we have. Our opinions matter, our opinions should be trusted and they should always be sought after. Although the Exec might be the 'big boss', it doesn't always mean that they have the best lateral approach to administrative tasks. With all the experience we hold, it would be crazy not to engage us and seek our opinion on certain things. We are at the end of the day, the closest person to the boss, and that is something that everyone, including the boss, should remember.