Create a study schedule with breaks built in. This doesn't have to be extremely rigid or require you to schedule all your activities down to the minute. Consider setting your schedule in terms of activities rather than time. For example, plan to study one unit of a class before taking a walk around your neighborhood, and then study the next unit.
Get some exercise. Taking a walk, going to the gym or jogging can all be great ways to exercise your body while your mind takes a break. If you are active in a sport such as boxing or tennis, work out on a punching bag or practice your serve. The more focused you are on your activity of choice, the better able you will be to take your mind off of studying for a while.
Try a mental activity that isn't related to your studies. Many students, inundated with textbooks and other materials to read, eliminate reading for fun from their lives. Pick a novel you have been wanting to get to and read a chapter or two for your study break.
Watch a movie or favorite TV show. Sitting in front of a television and simply watching an entertaining program can be very relaxing. Shifting from an active mental state to a passive mental state for even a short time can help you recharge.
Eat a snack. Low blood sugar or an empty stomach can make it hard to concentrate. Put down your books and walk to a coffee shop, convenience store or even a vending machine and pick up a snack to fuel your studies.