By my experience, the best way to get rid of that single cramp is stretching the muscle by an external force, for example with your hands or against the floor. But carefully!

How to get rid of the tendency to cramps - this is difficult. I always thought you should just take magnesium, but recently I read that cramps can be caused not only by lack of magnesium, but also by lack of calcium, iron, vinamine E or other important minerals and vitamins...

It also can be a certain movement or a certain pair of shoes that tends to produce cramps.

Some people jump up and down on the floor.

Put on a dry pair of thick socks when you come home.

I used to suffer terribly from night cramps after playing football. Someone suggested bananas helped with cramps so I tried one before playing and it seemed to work for me.

This sounds like a circulation problem. High intensity exercise will take care of this over the long term, a short term solution is only delaying recurrence.

What causes cramp: Low electrolyte concentration in your body (most common). One lecture I attended mentioned that this made it difficult for the body to send nerve signals, resulting in the cramp (I made the assumption this was from confused signals, hopefully someone can correct me/sent me straight on this).

Effective ways of getting rid of cramp: Good hydration. Water will normally do the trick, however if you find it hard to consume enough water, electrolyte mixes (such as gatorade/powerade) are another solution, don't drink these during exercise unless you will be going for more than an hour or so. Electrolyte drinks are very good for quickly dehydrating yourself after exercise.