I recall reading that the upper part of your arm (towards your wrist, but not the risk exactly), determines the strength of your grip. That is why a lot of people can work out and never increase the muscles in this area, because lifting a weight does not necessarily strengthen your grip. Things like carrying dry wall (or other giant slabs balanced mainly by your hands), mountain climbing, and strange workouts can increase the size of the rest of the arm. Straight lifting will give you a very bunched up muscle in your forearm, all the muscle towards your elbow.
I recall reading a good exercise for grip is to take round metal barbell plates and grip them in your hands with 3 fingers (index, middle and thumb) and then do squats while holding the plates and keeping your arms down by your sides. Your arms don't move but your legs do, the 3 fingers will quickly stress your grip.
Sorry if I can't be of much more help.