So here it is, I'm fresh from a year with my professional company, and here is a list of things you should know:

-don't work for nothing. If you have to stop going to rehearsals because you won't be able to pay rent, there is a problem, especially if the company is holding out on you
-keep your chin level with the camera when taking pictures. Your adam's apple is SOO not that important
-don't let them keep a bad picture, tell the photographer to delete it, cause I GUARANTEE that will be the pic that they use forever and always
-don't be the schmuck that isn't in rehearsals
-ALWAYS be ontime. Know your steps
-Help others
-don't complain
-drink water.

Feel free to add any advice you can think of.
-Julian