To answer this question accurately it is important to know the age of the athlete. The intensity and style of training can be markedly different for a high school or college athlete than it would be for a younger athlete.

That aside any type of functional training that incorporates leg type movements similar to ones they use in the game are important to use. Also, make sure to train areas of the core (center of the body - abdominals, gluteus, chest, back etc.) and always both sides of the joint. To many will train hard on their quads and neglect hamstrings and gluteus causing an imbalance and higher risk of injury.

Here are some links:
http://www.youtube.com/watch?v=MLBAbqEZIeU
http://www.youtube.com/watch?v=jOdbSh3IVSc
http://www.youtube.com/watch?v=J3ps5V0dtgU (watch hole thing)
http://www.youtube.com/watch?v=opuA4Ej2GDs

Kirk Mango
"Becoming a True Champion": http://www.becomingatruechampion.com/
"The Athlete's Sports Experience": http://www.becomingatruechampion.blogspot.com/

For most sports, including baseball, ti increase strength you should be training movements rather than isolated muscles. There are a lot of plyometric exersizes that will increase strenght, movement, and explosiveness. http://www.sport-fitness-advisor.com/plyometricexercises.html

I coach a 10U team, and we use a portion of each practice for conditioning and core strength training. For legs we do jumping squats, lunges, mountain climbers, hurdle jumps over our gloves, zig-zag hops down a baseline, and duck walks around the bases.

You may want to look at a website www.youthfitnessmag.com It's new and they probably would write you a workout. They are doing a lot of new stuff.