Take breaks while practicing. A good rule of thumb is a 5 to 10 minute break every hour. Some individuals need to take a break every half-hour. Participate in a regular exercise program. The ideal form of exercise for musicians would provide good aerobic training, involve the upper body, and be unlikely to result in injury.
Swimming is an excellent all-around exercise.
Aerobic dancing classes, rowing, and cross country skiing can also be beneficial and low-risk. Nautilus equipment can provide good conditioning, but be sure to get proper instruction in its use. Exercise is a good tension and stress reliever, too. Obviously, you should work closely with your teacher to develop and maintain good technique. You may wish to talk to your teacher about taking lessons in the Alexander or Feldenkreis techniques.