Calcium and/or divalent cations. They are needed for muscle contraction. This question comes up a lot, many of us have trouble with cramping. I think we all have our own answer. For me, calcium. Not Tums or other meds, calcium (why take a medication you don't need?). I've noted that a lot of sports drinks and electrolyte replacements have calcium as a major ingredient.
What works for me:
1) take a calcium/Magnesium/Vit D tablet right before a ride, and then every 50 miles or so (I like Citrical, but not sure any other brand wouldn't be just as good)
2) hydrate
3) Stretch - lightly before a ride and do a good job after the ride to lessen the cramps next time. I also stretch on the bike - in a downhill section pedal backwards to a place where you feel a stretch on the muscle in question if you can (for me it's my calves, so I just push the heal down while using the other leg to push on the pedal to give resistance).

Another thing you might try is endurolytes - many of my long distance riding guy friends swear by the stuff. Interestingly, it contains a lot of calcium. But, it has a lot of other things in it, too.