Based on a lot of feedback from many artists, this is what most artists I know, as well as myself, look for in an assistant:

1. An assistant should make the key (or the artist who is using them) look good. Whatever it takes, help with whatever the artist needs, and always watch to see what is needed, don't wait to be told what to do.
   
2. Never be late to a job! I am always about 20 minutes early; I sit in the car and read or make calls. This way if there is trouble on the way I will still be on time. This is the least an assistant can do.
   
3. Never handout your card, or make buddies with the art director or production staff. This will insure that you will not work with the artist who hired you again. I know you are looking for future work, but do it on your own time, and it is best not to try and take it from someone who was nice enough to get work for you.
   
4. If you are sitting relaxing while the key is working then there is a problem. Unless the key asked you to please sit down and take a break, you should be working at least as hard as he or she is.
   
5. Invest in a makeup kit (all the supplies you need to work on your own). If you are lacking supplies, you will always be just an assist - no one will want to refer you for a job if you do not have the tools to do the work. I have had many "artists" call me and tell me that they are artists looking for work, but do not yet have a full make-up kit. That is like a photographer without a camera - no one will take you seriously if you have not at least invested this much in your career.
   
6. Never bring anyone to a shoot with you. This is unprofessional.
   
7. Never bring a camera to a shoot. This is also unprofessional, unless you are shooting for continuity.
   
8. Never tell the models how to pose or tell the photographer your ideas about how something should be shot. Even the key would not do this unless the artist/photographer relationship was well established and this issue has been discussed at length between them.
   
9. Established artists are where your best work can come from, so treat them well.
   Never talk poorly about another artist, it will come back to haunt you. The last thing you need is any artist (even a bad one) to harbor resentment toward you. The more friends you have in this business the better you will do. Try not to look at other artists as your competition, look at them as you colleagues and you will do a lot better, and be happier, you may find your best friends in this business. I know I have.