Before you get ready to swing for the fences, you need to get a good, tight grip on the bat and be able to hold it in a still position. That takes strong forearms and triceps. Performing wrist curls and reverse wrist curls will help you wrap your hands around the bat and keep it steady.
To keep your shoulder stable you can increase your shoulder strength and stability by performing chest presses with a ZoN Balance Ball and ZoN Dumbbells, or by doing internal and external rotation exercises with a ZoN Dumbbells. This will help with bat control, too.