MLB 08: The Show Cheats

When in Season Mode, you can raise anyone's attributes by going into Player Management, select View Roster and click X on the selected person to give the attributes to. Scroll to his Batting Attributes and put left or right to increase.

When you draft a player from the Amateur Draft, you can change his age and change his attributes. Max out all of his attributes and change his age to 18 after every season. He will never retire because of age or ability.