For almost ten years, I couldn't finish any race over a 5K--and in the past couple years, not even a 5K--without getting such a severe stitch on my right side that I'd have to bend over double, quit running, and then have extreme soreness for days afterward.
I was ready to quit.
And then Tom Cotner, masters runner Tony Young's coach, gave me some advice:
1) Do leg lifts, versus other ab work, to strengthen the deep abdominals. Start easy (10-20) and work up to 100-150. Oh, and use bent knees so you don't hurt your back.
2) When you feel the stitch coming on, use a 4-Step breathing method. Breath in twice in rhythm to two of your steps (right foot, left foot--or vice versa), then purse your lips like you're blowing out a candle and breathe out for two steps. Continue breathing like this until the stitch abates.
I added one other element: I take two Advil before longer races. Hey, women use this for cramping; I figured it couldn't hurt for me (now, I know there are dehydration issues with Advil, but anyone, like me, who can't finish a race because of cramping will take the dehydration!).
Okay, so here's the happy ending:
I can race again. I don't get race-ending cramps. And with the extra energy my body now has available (versus what it used to waste on the cramp), I'm running times equal to what I ran back in my twenties (am 42 now)!
So good luck.
Hope this helps.