Back to a guitar-related subject. Last week I bought a device, that seemed useful to me - the Planet Waves (or D'Addario if you wish, same thing. D'Addario uses the Planet Waves brand for everything not string-related.) Varigrip. It is a little tool, that will help you increase the strength of your fingers, giving more control over their motion on the fretboard. It features 4 spring-loaded pistons (one for every finger) with tension adjustment to which will help you to move to the next level when your fingers get stronger. The device supports a variety of positions that mimic the most common playing situations. Check it out:
The whole body of the Varigrip is made of plastic, that feels solid enough to last for a while. The grips are rubberized to prevent your fingers from slipping. The rubber cap on the back side (opposite from the pistons) can be removed and placed on the pistons. Done so, you'll find a surface that looks like this:
It is supposed to mimic strings and help building or maintaining calluses. Can't really tell if it works, since I already have mine. Also I was under the impression, that the high strings are harsher on the fingers, than the low ones. I might be wrong, but anyway it's nice they even thought of it. Overall I find it very useful. You can exercise everywhere you can't take your guitar to like at school, at the office, on a bus ride, in a train, plane, automobile or the strip club. I like it.
P.S. I am not affiliated with D'Addario, I just like it.