Q. What Should I Do To Swing Harder For Baseball?

I play competitive baseball and have been weightlifting for about 2 years. I know athletes shouldn't train like bodybuilders but should I still only workout each body part once per week? My goal is to be able to swing harder and have more power.
I don't believe you have to train each body part once a week. The reason is for most of the sports programs you have a low level of volume each training session for specific body parts. For example, I will have most athletes Snatch, Clean, Squat, etc. all in one week.
What is more important is the volume and intensities that you incorporate. You should not be wasting your time with exercises that are not going to be beneficial to performing better on the field.
Hitting for more power is often misunderstood. The muscles that cause the mammoth home runs you see are not the arms. The arms are actually much weaker than most of the bigger muscles in your body. The muscles responsible for great power in hitting are the hips and torso. This is why even baseball players can benefit greatly from incorporating variations of Olympic lifts.
These movements teach the athlete how to use their hips properly and develop a great deal of explosive strength. The other aspect is great torso strength. I am currently working on an article demonstrating abdominal exercises that will create the type of strength in the abdominals required for sport. In sport, the thicker (not fatter) torso is preferred to dissipate force and to create huge power outputs.
Recently, Dr. Mike Hartle wrote a complete series of articles on how to use a sledgehammer to create the torso strength and power that would make most fitness enthusiast shake!
As far as splitting up your program, I suggest you think of movement instead of individual muscle groups. For instance, don't think when you are going to do your arms. Think of all the exercises that are involved with elbow flexion and extension, this would include all pushing and pulling movements. This also goes for looking at torso and leg work.
Instead of thinking of a "six pack", which is really the result of low body fat levels, think of all the motions that are important to strengthening the abdominal wall. This would include trunk flexion, side bending and rotation. If you only focus on one aspect you are neglecting full training of this region. For leg training you should concentrate on a few movements and their variations.
This would mean you focus on hip extension and knee flexion movements. What would this mean? Many exercises such as deadlifts, good mornings and all the Olympic lift variations. Knee flexion would include back, overhead and front squats, lunges (all variations) and single-leg squats.
These are just a few of the many exercises that would be appropriate for baseball training. Special emphasis should also be placed on upper back work to counteract all the throwing actions.
I like to think of focusing on several core lifts. This would be any Olympic lift, deadlift, squat, even bench presses. The rest of the exercises are supplemental lifts that compliment the needs of the specific sport. This would mean improving certain motor qualities such as explosive strength, speed-strength or maximal strength.
However, remember your program should also incorporate sprint work and range of motion drills. Don't think one-dimensionally just as in the weight room.