I used to get charlie horses pretty frequently. In addition to paying attention to electrolytes, salt, bananas, all the stuff above -- I also got into the habit of stretching my calves before bed.

Also, if your sleep position allows, try not to leave your feet in a pointed position as you fall asleep. For example, if you sleep on your stomach, you could scoot down and hang your feet off the foot of the bed. I think the idea is to not leave the muscles in a contracted position if there is a likelihood of cramps.

Sometimes the charlie horse just comes on all of a sudden with no warning, but after I tried the stretching and trying not to point my feet, I started to feel a few twitches before the full-on cramp. Sometimes the twitches wake me up before the muscle totally cramps up, and then I can quickly try to stretch the muscle/rub it before the cramp gets too bad.