Stretching before ballet class may help loosen and warm up the muscles, making dancing easier. Dance author Lauren Warnecke divides stretching into two categories: static stretching and dynamic stretching. In static stretching, the dancer picks a position at the end of his range of motion and holds it for as long as possible. This can help you practice positions and movements such as splits and high leg extensions. Dynamic stretching, on the other hand, is a series of active movements that increases the joints' range of motion. Examples of dynamic stretching include leg kicks, grand battements, and arm swings.
Rehearsing routines before class is a good way to have them in your head before the class begins. You don't have to literally practice the routine at home -- it can be dangerous without the proper space to practice. But know what you are going to do in the next class, and rehearse it mentally between classes while you're doing other things. Play the music you'll be dancing to and envision the moves you'll perform.
Water is hugely important for ballet dancers because they sweat so much during lessons. For dancers, the recommended eight glasses of water may not be enough. Drink as much water as you can before your class (and don't forget to go to the bathroom). Drinking beforehand will keep you from having to run to the water fountain during ballet class, and it will also keep you alert, energetic and better able to pay attention to your instructor.
You may have stretched fully and consumed several bottles of water, but you can't have a good ballet lesson without being mentally prepared. Even full-time dancers have issues outside ballet to think about, and it can be hard to pay attention in class when your mind is on other things. So take some time to unwind before class: arrive a few minutes early, put your dancing clothes on, and sit quietly for a while, letting your mind calm down. Take a few deep breaths to increase oxygen flow to your brain. It doesn't take long, but clearing your mind makes a big difference in how you will approach the lesson.