There are those weird squeeze clamp things that are designed JUST for finger and hand strength. I know a few guitarists who swear by them. They're small, so you can also take them with you to work out when you're driving, or sitting in bed, or what have you.

I've also heard a few people (not many, mind you) who swear by those little sand filled 'stress balls'. Also, though I personally don't see how they'd help, I have one friend who is convinced that the wrist weights that joggers use help - he thinks that they help increase the blood circulation and the endurance of the arms. I don't know if it does or not, but he says they do.

M