I wanted to add something to the cramping issue (not sure if it was mentioned earlier, because I briefly skimmed the thread, but I originally wanted to post it and got delayed in confirming it; btw, this blew up into a huge, informative thread, you guys are very well rounded) - the definition of a cramp is "A sudden, involuntary, spasmodic muscular contraction causing severe pain, often occurring in the leg or shoulder as the result of strain;" scientists are not 100% sure of why it occurs but there's 3 areas of thought on the cause: 1) nutrition; 2) hydration; 3) conditioning. you guys have touched on all 3 but I don't think anyone mentioned proper stretching and warm-up (and this has to do with conditioning). a substantial percentage of injuries are caused because of a lack of this. as lame as it sounds, take 10-15 minutes before a hard ride and stretch (both static and dynamic; if you're going to ride hard or race, spend more of your time doing dynamic stretching, this "wakes up" your muscles and should be specific to the muscle groups being used). also, an often overlooked part of conditioning, gradually decrease your workout as you near the end (in other words, at the end of a hard ride, take an easy 10-15 minute ride to gradually decrease your heart & respiratory rate; pedaling at 30-40% of your maximum effort for the first few minutes and gradually decrease from there).
Now as to what normZurawski was saying about the hill repeats. he's 100% correct and he's referring to "interval training" which has been shown to be very effective (there was just a pretty good article on it in either this month's or last month's Mountainbike Action) for improving stamina, speed, and conditioning. it's actually the fastest way to increase your speed and stamina in the shortest amount of time, and it's also a way to get in a tremendous workout in a short period of time (in other words, doing 6 hill repeats will consume say 20 minutes and covers X miles, this is will be the equivalent of riding 60 minutes and doing X times 3 miles - these figures are just for illustration, I'm not sure of the actual conversion but it is documented that interval training condenses a longer workout).