Take breaks from studying. If you feel your stress levels rise, walk around the library or take a few breaths of fresh air outside.
Exercise. Getting your blood flowing releases chemicals in your body called endorphins that make you feel good. (see refs 1)
Stretch or practice yoga while breathing deeply. Stretching your muscles feels good after sitting for long periods of time while studying. Breathing deeply helps the mind relax.
Remove unnecessary obligations from your schedule. The most important thing for you is to study. Don't worry about the rest.
Eat on a regular schedule. If you plan to be away all day, pack a lunch. Eating regularly helps you maintain your energy levels and stay calm.
Trade massages with a friend or significant other. Massages are allow your muscles to relax and your mind to feel peaceful.