Pause the game and hold R + Ctrl and press the following:

All Levels:
Press Up, Down, Left, Right, Right, Down, Left.

Infinite lives:
Press Down, Down, Up, Down, Right, Down, Up, Up.

All Characters:
Press Left, Right, Left, Left, Left, Up, Left, Left.

All Extras:
Press Right, Up, Up, Down, Right, Right, Down, Right.