I get cramps in my hand also, but it is a lot better now that I have modified my technique for holding the neck during barre chords. I am a beginner and still working on my technique, so I would appreciate any one else's input.
The bad way: Looking at my left hand from the top, I used to squeeze the neck of the guitar between the bottom right side of my index finger and the bottom left side of my thumb, making the thumb point in the same direction as the frets. I used to quickly cramp this way because the muscles used to move my fingers laterally are relatively small and weak.
The better way: Now I point my thumb down the neck toward the nut. This allows me to still use the bony parts of my index finger to make the chord, but also allows my to use the stronger muscles in my hand to squeeze the neck.