In no particular order:
1) Professionalism - the ability to respectfully interact with people at all levels of the company in an open, friendly and polished manner. Show up on time. Keep his comments between the two of you - his personal & professional life is not for you to share. You are representing your manager - e.g. your manager may not care to do something and it is your job to gracefully decline on his behalf.
2) Proactive - the skills to forsee and provide whatever (and bring in whomever) he will need to prepare for and at meetings.
3) Organizational skills - definitely. Now, where is that slip of paper he handed you last week?
4) Interest in learning - about his job and the company, industry, etc. Keeping up on business news related to the above as well as researching new vendors and learning new software tricks. Have an inquiring mind. When I go to a new restaurant, I can't help but wonder if it would be a good place for a team outing, lunch meeting location, etc.
5) Networking ability - know who his key players are, get to know the assistants and the assistants other bosses, etc. Attend networking events even if you are the only assistant, get yourself out there and you may run into someone that will be a key contact for your boss - knowing you may help them get a meeting setup.
Good luck!
Edited by kellinm on 27/04/07 05:14 PM.