I reckon I've got one of the strongest wrists around, well after Dibble that is. It still happens to me durring the first couple of sessions untill I get myself relaxed.
Powerball for the strength, mine's got flashy lights in it too.
The thing from Golds Gym will increase grip strength, but for a rider you need rotational stability etc as well, which the 'ball gives you.
I'd also recommend a lot of stretching exercises for the tendons in the forearm as well. These are the bits that really tighten up after consistent clutch use.