Drink LOTS of water. I drink about 1.5 L during a 2-2 1/2 hr class, then another couple of liters after I get home.
Eat good, healthy food. Eat light before you train, and a good meal after class.
Stretch a lot after class. After I get home I usually stretch for 1/2 hour to an hour. If you don't, you'll be really sore the next day, especially if it was a tough class. Stretching also prevents injuries, and you'll feel more relaxed for the next class.