Ooooooowwweee. I hate those cramps, there a bastard to get rid of.
I take the magnesium tabs normally at the first chance I get during a ride, normally the first pit at an enduro, but for a XC I take them right before the start of the race, and at each pitstop along with a leppin squeezy gel shot. And drink lots and lots of this endurance drink which has amounts of magnesium phosphate in it, but the pills work much better.
Like I said, never had them while riding, normally I tire after about 4 hours of riding, and by that time the race is finished anyway. But I have gotten those nasty thigh cramps before on the way home, and had to do the mad dance stretches on the side of the motorway before :D

Thats the dance!I'll get some tomorrow,thanks.

You absolutely take the tabs before a ride.

Revisiting stretching... My cramps are usually in my foot, or my hand, or finger. Occasionally in a big muscle, but mostly in muscles that are not really ones that you think of stretching. My favorite muscle cramp is in my chin/neck/jaw. I don't know what it's called, but it's that part under your tongue between your jawbones. It's awful. Fortunately, it'll hit my foot, or radial muscle before that one.

Ohh i get that one when i'm yawning sometimes, man that hurts! have to drop everything and massage under my chin to make it go away, dont know how i would cope if that happened whislt riding!!

After some reaserch, and talking to a nutritionist I have some info. Apparently the human body likes to be slightly alkaline in it's PH balance, preferring 7.3-7.4. I took a reading and am at 7.1. The nutritionist works with several long distance athletes and says that there are several ways to keep your PH where it should be, most of which involved eating lots of green vegetables, especially spinach and broccoli, and cutting back on meat. I'm not keen on cutting back too much on meat, but I digress.

She suggested a product called Acid Zapper acidzapper.com. Using this product apparently helps get your body to that ideal, and they claim you can't overdo it, which is good, because I'm overdo it boy.

I ordered some of their tablets and granules for the camelback. I'll report back when I can test it.

that may explain why cramping is never an issue with me. vegetarian for 22 years. no cramps, but a plethora of other issues :)

Initial report.
I used the Acid Zapper this weekend on a ride that should have certainly made me cramp. I did not cramp. Didn't even come close. Pretty excited about that. I'll post updates as I get more experience with it.

I have heard that magnesium helps you absorb other minerals that you need. I do know that you must start frequently drinking water a day and a half before your event. If you wait until your thirsty you will never catch up.

Follow up to all this. After several months of using Acid Zapper, I have found that I can still cramp if I let myself dehydrate. I have also discovered Clif Shot blocks. They are much easier to choke down than gels. They also seem to help with electrolyte replacement.
Alas, nothing seems to be the "silver bullet." Stretching is crucial, as is hydration. Good thing for me, the combination of all of this has resulted in much less cramping. The last cramps I had were at the Idaho Qualifier after being sorely dehydrated and overworked!

I have read that when muscles are simply pushed beyond their "normal operating range" they will cramp. The theory being that training and conditioning can extend the "normal operating range". Assuming hydration and mineral replacement is kept up, the performance of the various muscles we use when riding will improve hugely the more we ride.
Roost, have you noticed if you are cramping less as the season progresses?
For myself anyway, I know at the start of the season, even though I drink heaps, I get lots of cramps - but after more regular riding through the summer, the cramps "barrier" is usually pushed out to say 4 hours.