Level select:
Press A, Y, A, Y, X, Y, B, A, R, L at the option screen. The screen will shake to confirm correct code entry. A level selection screen will appear.

Ten continues:
Press B, A, Y, A, X, A at the option screen. The screen will shake to confirm correct code entry.

Twenty continues:
Press Y, X, B, X, A, X, L, R, A, X, A at the option screen. The screen will shake and the number "20" will appear below the "Options" selection to confirm correct code entry.