DON'T make and leave a mess of your stuff in his/her truck.
DON'T rearrange the trainer's things ... it is his/her truck, after all.
DON'T take your sweet time in the truck stop so he/she has to always wait on you.
DON'T stop the truck unless you really have to. Be sure you know where you're supposed to be changing highways, stopping for fuel, etc. so you don't have to stop and look at the map. BUT if you aren't sure, stopping to look is preferable over going the wrong way!
DON'T turn the music up very much when your trainer is trying to sleep. There's very little sound barrier between the cab and the sleeper and even if you leave the volume low, it can still be heard in the bunk pretty well.
DON'T turn the CB radio up very loud, either. Turn the squelch high. Try not to talk loud, if you use it. For that matter, you may want to ask if the trainer minds if you talk on the CB.
DON'T say to the trainer, "Well, that's not the way we did it at driving school."
DO maintain good hygiene.
DO your part ... go on the docks with the trainer, unless he/she says otherwise. Get out of the truck at fuel stops and help clean the windshield, thump tires, etc. Help to slide tandems.
DO ask for clarification if you don't understand directions you're given.
DO ask the trainer what they expect of you – don't be afraid to ask questions! If you don't understand how to do something, the trainer will strongly prefer you ask rather than to have you do it wrong.
DO find out if the trainer smokes, if possible. If you do and the trainer does not, don't expect smoking privileges in his/her truck!
DO pay attention to what the trainer does -- even seemingly mundane tasks such as fueling the truck and checking in with shippers/receivers.
DO show respect and courtesy -- even if they don't... probably the hardest advice to follow.