Dr. Murray Weisenfeld, in his book The Runner's Repair Manual, gives several stretches that will help you avoid a groin pull. These are illustrated in my page of pictures of stretches. Do the toe press and the inner/outer thigh.
According to Weisenfeld, if the pain moves from the upper groin area to the lower groin area, you may have an ischeal fracture of the pelvis, and you should see an orthopedist. In addition to the suggestions given by Weisenfeld, go to the Sports Injury Bulletin Groin Injuries and Groin Pull and Strain web sites.