Marvell - I believe that a coupel years ago, in a discussion very simlar to this on TalkMagic, one of these was recommended -
Gripmaster. They build hand strength in a variety of ways - I think the idea is once you've got the muscle memory, you can build the muscles to hold more, or hold what you are already holding more efficiently.
It probably wouldn't do any harm to any manipulations, come to think of it. Cards, coins, sponge, rope... I might treat myself to one...