I don't link to any commercial product or any other site because Revolutionary Tennis is about you, not me.  I came across this product that strengthens your grip.  I feel it does this job much better than a hand spring, squeezing a racquet ball or small balloon filled with sand because each finger is reached evenly and there is flexibility in its use.

Tennis players need to work on their hand strength, it leads to better quality strokes.  This product is not expensive, and it has a variety of uses.  Revolutionary Tennis has zero $$ gain in this referral.

It's called the Benji T. Gripster.