Right before your are about to die, press Start on controller two and you will get more lives.
Infinite Continues
When you've used your last continue and the ''continue'' countdown comes up with the words ''game over'' flashing in your corner of the screen, just hold the triangle and select buttons and press start. You'll get five additional continues. Repeat as needed.
Level Select
At the title screen hold Up + Left + Select and press Circle.
Speed Up and Slow Down
While you're playing the game, press START to pause. To speed up the game slightly, unpause the game while pressing Triangle+R2. To slow down the game slightly, pause and then unpause while pressing Triangle+L2. You might have to do this a few times to get the full effect.
Two Player Battle Mode
Enter the level select code. One of the avaliable options should be two player battle mode.