Show up to class on time, the warm up is important to strengthen and develop your muscles to do the moves.
Turn your cell phone off
Be supportive of your classmates. If a newer member is struggling it is okay to try and help them out; it might help you understand the move better too.
Don't say "I can't....", you can do anything you set your mind to. You may not have the skill to execute the move at that point in time just the way you want to but... with time and effort you will get it. If it was easy why would you have to pay me money to teach you?
Don't use your body type as an excuse..."My body doesn't do that because I'm bigger than..." or "I don't have any hips so it just doesn't look the same". All women of all types bellydance. The top professionals range from tomboys who are 5'1" to voluptuous sirens who weigh 220 and every other variation you can imagine. I will get a bit irate if you blame your body size on any difficulties you may have in class. You can do anything.