Although discontinued, the Wrist Wrench is a phenomenal training tool designed to put pressure on your fingers - simulating the opening of your hand as you cup. There are many knock-offs of this tool available, or you can make one yourself relatively easily.
Strengthen your hand by increasing the size of your weight and pull-up bars. Other brands may be less expensive, but they all generally work the same. You can even use pool noodles to get the same effect!