This is an easy program for starters. Do it 3 times a week. Note that it only should be done by players with stable mechanics, I would not start before 12-13 years old. Before that work on mechanics and other stuff.

Here is the program. Do it 3 times a week on non conscutive days:

1.extensive warmup

This is extremely important because otherwise you can get injured. Especially make sure to warm up obliques and back, I once strained my oblique doing overload/underload not warming up properly and missed half a season. So REALLY warm up.

2. The tire drill

http://batspeedfitness.com/?p=40

Do 3 sets of 10 swings at full force. Two sets of the strong side and one of the weak side to balance it out.

3. Back step turn drill

3 sets of 6 swings as hard as you can.

http://batspeedfitness.com/?p=59

4. Overload/underload training

2 rounds of 5 swings each with the light, regular and heavy bat ( 30 swings overall)

http://batspeedfitness.com/?p=31

5. Jumps for the legs

a. 2 sets of 8 vertical jumps

b. Two sets of 10 skaters jumps