QUOTE(snatchingthepiano @ May 3 2008, 06:32 PM)

I wouldn't do the tennis ball thing that jacobflutepianoorgan mentioned. Seems to me that you'd probably just tire your hand out and if it somehow did offer results, they would probably take a long time to manifest.
Exactly why I haven't tried it!

I think the only way to strengthen your fingers without injuring them is just by playing more, and it will happen naturally. Using a tennis ball might give you RSI as well, which means not playing for ages!
I use the changing rhythms thing that snatchingthepiano mentioned, and it is very useful. You change the rhythm to a dotted one, and try lots variations, and then when you go back to the original rhythm, it is usually easier.
Good luck with the chromatics!

Jacob.