If you want a strong crushing grip, ditch the grip master; it is simply too weak to build any significant finger strength, although I can definitely see the rehabilitative uses of it. I like the Ironmind captains of crush grippers, but there are some others out there that cater to developing truly strong grips.
More importantly, consider getting some rubber bands or something and doing finger extensions. Put the rubber band around your fingers and extend them repeatedly. This will help keep the balance in your muscles and may help prevent injury; they help greatly for heavy grip training and a daytime job of computer work for me, and it is pretty inexpensive to find some big rubber bands to use.
UberGoober
6-22-08: 7.5' BPEL, 7' NBPEL, 7.75' BPSFL, 5.25' EG
Goals: 8' NBPEL, 6' EG, 21' NBPSFL