To get unlimited lives, pause the game and press X, R, L, A, A, X, Y, WHITE, B, L, R, BLACK, UP, UP, DOWN, LEFT, RIGHT.
If you leave and re-enter an area (go through a portal then back into the stage you just left) all extra life hearts respawn on the level. If you go to say the mansion level you can get 2 extra lives per trip there (on second and third floor of the mansion). You can do this unlimited times (or until you get tired of doing it at least).
It takes quite a bit of practice to master the course, hit the "afterburner" rings and line up the jumps PERFECTLY. Any mess ups at all can cost you the race. Jimmy cheats, so using these shortcuts will really help.
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