Stitches can come on for many reasons, electrolyte inbalances is one (as Jason has explained above) try drinking a sports drink or orange juice diluted 50/50 with water which will rebalance your electrolytes and help if that is the issue.
Posture can be another reason, especially if it's a shoulder stitch. Work on making sure your back is straight, hips are inline, knees are below your body and not running ahead of it. Then make sure your upper body is relaxed as tension can create cramps. It can take a while to get used to running without cramping up.
Fatigue though is another big reason. The key to this is keeping tabs on how you feel and stopping yourself before the stitch becomes a stitch -you do get subtle warning signs. If you feel a stitch coming on, slow down and stop, stretch a little, don't totally cool down but get yourself to a position where the to-be-stitch feels pretty much gone, then go back to running. Once you have a stitch your on a losing streak as to trying to get rid of it as it sort of leaves a mental mark behind so even when it is gone, your posture has already altered to expect it to come back. Doing this can end up with a stitch elsewhere (eg shoulder!) and then you get nowhere fast.
