The best method is to train your body so that the strength on tap in the musculature is available instantly with maximum velocity. The result of strength X speed is power. And, in our minds, the best way to develop pure power is to work on both components in a program that's been called 'Pyramid Training'. You combine the standard resitance of your game bat or an offical baseball, with swngs or throws in which you must work against heavier than normal resistance, plus reps in which the wieght is lighter than normal (lighter than expected) and so the body can generate speed woith less resistance.

Think of it as a progressive workout...

For batters: with standard weight of say 30 ounces, light would be 27-29 oz and heavy is 31-34 up to 40 oz.
For pitchers: official ball weight is 5 ounces, light is 4 oz, heavy is 6 and more, up to 12 oz with some programs going to 21 oz and beyond.

The sequence (after warm-up) would go standard-heavy-light-standard.

For hitting, 25 standard bat swings then 50 heavy, 50 light, 50 standard for full workout.
For pitching, 10 standard-weight pitches then 20 heavy, 20 light, 20 standard.

The actual number of reps would build during the season, just as 'heavy' resistance would build.

There is more to it than that. And it is used in other sports too. When football players push against sleds, or runners use chutes or tow ropes to be pulled faster or slowed down, that's overload/underload training - the pyramid.

Variations are possible. So in addition to using, say, weighted balls, reistance tubing or cable weights are an option. And in addition or instead of bats with weights, you could use a bat with tubing or fins attached. (Information on training products is in the WebBall Product Guide.)