Watch the Stretching Techniques online guitar lesson by Stu Hamm from Stu Hamm U: Tap Bass
I just can't stress the importance of developing a warm-up routine of your own enough. By taking the time to warm up and stretch out before you play you can avoid a lot of injuries and have a longer, pain free playing career.
Here are some of the exercises that I use to warm up. Use these as a starting point and come up with your own routine that you use every day before you start playing.