There is little known about this problem. I recently paid $60 to access a scientific article that had done significant research on the subject, and the conclusion of the scientists was “we don’t know what causes side stitches or how to treat them.” The article gave 6 different possibilities and treatments. A promising link with current research points to poor posture (not just when exercising, but in general, a condition known as kyphosis) as a possible cause, with the poor posture possibly adding stress to the lining of the abdominal cavity. By focusing on good posture, you might reduce the frequency of this problem. See http://www.ncbi.nlm.nih.gov/pubmed/20022301
I suffered from this problem for nearly 20 years as a runner, and have only recently managed to completely eliminate it from my running when racing with the following changes, adopting a shotgun approach from the article previously mentioned, and implementing all possible treatments in a single season:
1. A general increase and focus on run fitness, particularly at high intensities. It is possible that this raises the run velocity threshold for when the pain occurs.
2. Abdominal massage. 3-4 times a month I have been having a therapist massage my abdomen, intercostles (rib cage), psoas, and lats to both relax the abdomen and to reduce tension in the torso, which can help my posture. I think that this is the #1 reason the issue has disappeared.
3. A product called Myomed on my abdomen before each race. This is probably not helping me too much, but it has been part of an overall successful system.
4. Deep belly breathing during the bike part of a race to relax the abdominal cavity. I did this in previous years with only minimal success, and so it may not be a significant influence.
5. A product call ASEA that I take daily and before each event. This is very unlikely to be the reason the side stitches have gone away, but I am too superstitious to stop taking it, since I stopped having the stitch when I started taking it.
6. Focus on good posture all day at my computer. Because the side stitches went away before I started my posture focus, this is also not likely what mitigated the stitch, but it may be another way to keep it from happening again.
Because I introduced almost all of these all at the same time, I can’t isolate which one, or combination or items, solved the problem. My guess is posture, and items #1 and #2 above are the most likely solutions.