Sometimes when I am playing the guitar standing up my thumb on the hand that holds the neck of the guitar starts to cramp. How can I mprevent this from happening?
Most of the time when a problem is related to playing while standing it means that the guitar is slung too low. Usually it's wrist pain that results from twisting the hand under the neck, but I can see how that would put strain on other parts of your hand as well. See if wearing the guitar a little bit higher helps.
As it's the humb I suspect you tense it up too much. The guitar should rest on it, you don't want to put any strength behind your thumb.
Hold the neck like you would an open tube of toothpaste. You don't want to drop it, but you don't want to squeeze the toothpaste out all over the floor.
Also, try not to bend your wrist farther than about 30-35 degrees while playing.
Ok awesome, thanks for the help. I'm gonna try it out.