Come to class prepared physically and mentally.
Arrive at the studio a bit early so you can get changed, do a few light stretches/ankle exercises/get your body ready - (which is different for each dancer). Have your hair, clothes and shoes prepared so that when class begins you are ready. It's very important that you do not miss the first exercises of class. They really do build the foundation from which your technique will come from- Barre is not to "warm you up" it is to build the technical building blocks in your body that all of your center, across the floor, variation, pas de deux work will come from.
Mentally, you must learn to put aside the worries, drama, chaos or even joy of the world and completely focus on ballet 100% when you are at ballet. If your mind drifts you miss information- a correction, a detail, even a whole combination can be lost when your mind wanders to other things. It takes practice and time- (and we all have days that this is easier than others) - but in general, TRAIN your mind to focus