MLB '08: The Show Hints

When you're in franchise or season mode and you can't win, go to the conroller select screen and move the controller marked P1 (it is normally on the symbol of the team that you're playing as), and move it on to the symbol of the team that you're playing against, and lose as them.

On franchise mode or season mode, when playing a game, go to the pause menu and select Controller Select. Move the Player 1 controller, which should be on the logo of the team you're playing as, to the other side, which is the logo of the team you're playing against, and lose as them.