I have just been reading this article on O'Reilly about preventing, whats called RSI (short for Repetitive Stress Injury). In simpler terms, RSI is injury caused by repeatedly performing a stressing action. A commonly seen example is use of computers. It stresses various parts of the hands. This can lead to damage to soft muscles and nerves. The article goes on to suggest some easy techniques to ensure that such body parts get enough rest so that the risk of RSI is reduced.
For computer users, there are several applications that can remind the user to rest periodically. One such example is Workrave (see pic).
If this interests you, here's some detailed reading material