When I played (haven't since '92
) I used hand crushers while sitting in class. Also took a 10lb. plate, tied a 5 foot piece of rope to it, and the other end I tied to a sawed off broomstick.
Put your arms out in front of you and roll up the rope around the broomstick by alternating your wrists up and down until the weight gets to the top. Then roll the weight down and repeat 'til your wrist/forearms/shoulders can't take anymore. If you can improve your wrist strength, you will increase your bat speed.
If you don't mind me asking, which league are you in (you can pm it to me if you want)? Been contemplating joining one, but I was gonna wait 'til I turned 30 and join an over-30 league.