From MLB Power Pros Wiki

In Success Mode 2008, there are 5 practices for fielders and 5 practices for pitchers. Each practice type can be leveled up to a maximum level of 9. The higher the level, the greater the point boosts are. Unlike practicing in MLB Power Pros 2007, the quality of the workout is decided by flipping 5 cards, which either show "Ok", or "XX". Five "Ok" cards grants a level up and the best gains. Five "XX" cards means that Chief was injured during practice and results in losing high amounts of points in addition to a drop in evaluation from both the Coach and the Scout as well as a poor condition.

Fielder:

• Weight - (Strength)
• Batting - (Strength and Technique)
• Run Speed - (Quickness)
• Fielding - (Technique and Quickness)
• Arm Strength - (Strength and Technique)

Pitcher:

• Weight - (Strength)
• Top Speed - (Strength and Technique)
• Accuracy - (Technique)
• Fielding - (Technique and Quickness)
• Breaking Ball - (Breaking Ball and Technique)

Keep in mind that all of these practices increase mentality, but only slightly (single digits)