Experts believe this jarring, hard-to-run-through pain is caused when the ligament attaching the liver to your diaphragm is overstretched. To prevent side stitches, try to exhale as your left foot hits the ground — exhaling as your right foot hits actually causes them.
If you're suffering in the middle of your run, slow your breathing and your running. If that doesn't help, stop, press gently on the pain, and push up. This moves your liver and diaphragm closer together, helping to remedy the overstretched ligament.