What's a good way to strengthen fingers while playing guitar?

They make a finger exerciser with different buttons that are spring loaded. I can't describe except to say go to a local music store and ask around, it was probably the best 20$ i've ever spent.

http://www.rei.com/product/478028

that's what i was refering to :D

I second Perchik's recommendation. The gripmaster helps. The best step for me was learning a complicated song that forced me to use all my fingers. You might be a few months away from that, though. I prefer exercises that use all the fingers, rather than exercises that only use the pinky and ring fingers.

The best way to get stronger fingers for guitar is to practice the guitar. If you are hurting, take breaks more often.

@kneck thats true, unless you're playing incorrectly and you create bad muscle memories, then you could be hurting yourself.

also correct hand position...my gtr instructor says that if you play with the proper technique your fingers shouldn't hurt while playing. Also maybe have the action on your gtr checked out as many beginners don't realize that the action may be too high causing undue muscle strain on the fingers...the gripmasyer does work but shouldn't be a replacement prooer technique. Scales and chords emphasizing all the fingers works for me.

sndfreq brings up a really good point that I'd forgotten. There are lots of inexpensive guitars that have terrible action (meaning the strings are too far away from the fretboard). These can be very difficult to play. If you think that might be the problem, you can go to a guitar shop and see whether the guitars there are a lot easier to play than your own guitar. The action on many guitars can be adjusted by a competent professional.

Hand position is important, too, of course.

I remember awhile back seeing finger weights that you put on your fingers while you played guitar. I haven’t personally tried them, but in theory, by using resistance in an action, you should be strengthening the muscles that achieve that action, so that might be a way to bulk up your fingers ^_^ Other than that, practice, practice, practice!