This might sound a little too pithy, but I found that the best exercise to develop hand strength for playing bar chords was simply playing bar chords... Started with working the full F bar into songs in C for instance... When I had that down, then I'd add another bar chord into the mix... Now I can play just about any song just in bar chords (Not that I imagine I'd want to)

The same hand exercises that are mentioned in any other similar thread here won't hurt either.. as you develop strength, you also want to make sure you're maintaining (Or even better, improving) flexibility...

But when it HURTS, stop! That's what pain is... your bodies way of telling you to stop!