I get them bad sometimes - almost always related to a lot of unususal exertion with my legs, combined with usually not enough water / electrolytes days and weeks ahead of the heat and exertion. Heat is not always required, but some form of leg exertion and not enough hydration is usually the cause. The heat makes it worse for me.

The instant relief to make them go away quick is drink real dill pickle juice straight from a jar of pickles.
PJ sport drink is not the same. Then I eat a couple bananas, eat 2 or 4 hylands leg cramps (CVS has the same pills for dam near half the price) and soak in a tub of the hottest water I can stand till they go away. by that time the banannas and hylands have kicked in. and i can go back to sleep. Before i get in the tub, or while soaking, I'll Slam down 3 or 4 bottles of water with electrolytes or sports drink 50/50 with water. Be careful that can keep you from sleeping if you over do it on the electrolytes or maybe its the B vitamins lot of them have for energy.

Preventative is to keep hydrated all the time, add electrolytes with the water as needed and a small bottle of Pom wonderful - (thanks Derek!) Goes a long way for prevention. I haven't tried the stretching but that makes real good sense.

Interesting story about the pickle juice - too lazy to look it up but the story from memory is..
Some college professor did a test on a bunch of people with cramps and determined the speed of pickle juice going into your stomach then your blood and then absorbed into your muscles to relieve the cramps would take totally way too long for the almost instant relief it can bring. His theory is the pickle juice somehow triggers some unidentified release of brain chemicals that produces almost instant effect of relief on the cramps.
Make sense to me because that stuff works way to quick for it to work about any other way.

I used to carry those prepackaged huge dill pickles that had a good portion of juice in the pack, snip off a corner with my pocket knife and slurp one down when needed.