Get yourself into a hot tub or spa ASAP if you're lucky to have one. Soak for 1/2 hour. This works for me.
To stop a cramp in the calf, extend the affected leg fully, stretching it out straight, then point your toes upwards and hold in the position. The cramp should subside within a few seconds. You can also extend you leg and stand on the ball of your foot whilst pointing the toes upwards.
Drink some vinegar. Maybe a half a small cup. It works immediately.
Take some table salt and put it in the palm of your hand about the size and thickness of a dime. Put it on your tongue and swish it around till dissolved then wash it down with several mouths of water, in minutes the pain is gone.
Eat about 1 teaspoon of brown mustard. It gets rid of the cramp immediately.
I noticed while watching a soccer game that they put ice on cramped legs. It works almost immediately if you can stand the cold.
I've found that bananas can have an immediate effect.
I take Magnesium Complete, and have also found that having a hot water bottle at my feet in bed is a great help. The bottle usually stays warm all night and keeps my feet and legs warm....no cramps.
I used the "soap between the sheets" remedy and it seemed to work fine.
As soon as you feel the cramp refrain from grabbing it. Hard to do when you are in a lot of pain. Instead grab the vein between your nose and upper lip with your thumb on the inside of your lip; index finger on the outside. You may not feel it but it is there. Pinch but not too hard. If done right it will stop the cramp immediately. If not gotten right away the cramp will be gone but the muscle will be tender for a while.