Pause the game and press X, R, L, A, A, X, Y, White, B, L, R, Black, Up, Up, Down, Left, Right.
Unlimited Heart trick:
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).
(supplied by: Wolvie26)