i have had chronic hammy problems too. each person has their own unique reason though -- especially since the "hamstring" consists of a bunch of muscles, but these problems include,
strength: not enough or bad ratio. when those quads are ripped (>1.5) compared to hammys = bad news
warm-up: this is NOT related to stretching, but people forget to really, well, warm-up. break a sweat (get hot), or wear tights even when you could wear shorts
flexibility: Marion Jones could probably sign her name on the sole of her shoe. strive for that, which means stretching after practice and/or the dreaded yoga. I'm not a big fan of the eastern stuff associated with yoga, but man do you get flexible fast
muscles all around are messed up: chances are, after torching your hammy, all the connections have problems. as previously stated, your lower back and abdomen should be attacked with massage as well as the injured muscle itself. get a good sports masseuse who knows what they are doing. if you happen to be in the Boston area I can recommend one.
dehydration: cramping can simply occur because the isn't enough water in the tank
not enough rest: everybody wants to bounce back fast. if the kid is always hurting himself, then maybe its time to TAKE A BREAK. Or just mileage for a month. Or a bit of both. Certainly it doesn't help to keep reinjuring it = permanent damage.