Although not all RSI can be avoided, properly warming up before one touches their guitar can seriously reduce the possibility of common repetitive strain injuries associated with playing the guitar. Athletes invariably stretch before any serious physical exertion - why should guitarists behave any differently? A web page displaying a series of Stretching Exercises provides illustrated examples of quick and relatively effortless warm up activities designed to limber up muscles, helping to reduce the risk of injury. To find more stretching exercises, designed with the musician in mind, check the Musician's Healthweb site.
Even after the muscles are somewhat more limber, it is important for guitarists to warm up with their instrument in hand, before diving into playing situations. This will not only help prevent repetitive strain injury, but sufficiently limber up a guitarist in order to play more accurately. Many guitarists will use simple scales, played slowly, accurately, and evenly, to warm up. The Dansm Acoustic Guitar Page has provided a series of warm up exercises for guitar, illustrating some finger techniques guitarists can use to get limber before playing intensely.
For guitarists further interested in preventing serious injury, I recommend reading the excellent, albeit brief, article, Repetitive Strain Injury Prevention Guidelines, written by Dr. Timothy J. Jameson outlining preventative measures for combating RSI.
Much thought has gone into the prevention of injury, as a result of repetition of irregular body movement. Several methods have been developed, including the Alexander Technique, that focuses on assessing and changing the amount of energy used in daily activities, with the goal of improving ease and freedom of movement, balance, support and coordination. The method is one which comes highly regarded within the music community.
When any guitarist properly warms up, it is not only immediately beneficial in terms of technique and fluidity. It also helps to ensure that they'll enjoy healthy and long playing careers, by avoiding problems like repetitive strain injury. It may seem like a bit of a chore, but in the long run, the benefits of warming up will be plainly obvious.