As for myself, since I've started playing again after a long layoff, I've been getting cramps now and then. I just shake my hands out and stretch my forearm muscles by interlacing my fingers, turn my hands out (thumbs down) and extending my arms. Another way to stretch your forearm muscles is by taking one hand, reach under the opposite hand, grab the opposite hand thumb with the palm of the grabbing hand against the back of the opposite hand, and, with both hands near the center of your chest, use the grabbing hand to crank the other; the grabbed hands thumb should be facing out, and the pinky facing in. There's a number of ways to stretch your forearm muscles that control the hands. I find keeping the forearm muscles loose lessens cramps.

When playing, my thumb can be behind the neck or on top, depening on what I'm doing, and where on the neck I am, but I've noticed I tend to cramp more quickly when I'm doing scales and my thumb is behind the neck; makes ya wonder if the old classical guys got it wrong, huh?