One of my favorite exercises to help develop forearm strength is to get a bar about 1in in diameter and 12-15 inches long, tie a thin wire to it, and attach a weight at the end of it. Grab the bar with both hands, one near each edge, holding it parallel to the floor at shoulder height, and simply start turning rotating the bar in your hands along it's axis such that the rope wraps around the bar as you do it, causing the weight to inch up. Once the rope is fully wrapped around the bar and the weight is at the top, rotate the bar in reverse. Repeat this three times. It might also help to get a pair of hand grips to use while you're watching television or otherwise or wasting time on the internet.

I think I've developed RSI in my left wrist from guitar playing, and now I have problems applying pressure to my wrist when the hand is bent backwards, such as when doing push ups. Any advice on how to deal with that?