I actually use the gripmaster in a way like dvenetian says - isolate pairs of fingers:

1= Index, 4=Pinky

1&2, 1&3, 1&4, 2&3, 2&4, 3&4 - it seems to works for my students. I actually recommend trilling with these pairs of fingers on a guitar, but in the absence of one you could do the same thing with a gripmaster, or even by using your other arm as a pretend guitar neck... people might look at you funny though...