When on the joystick select screen, type in "OPERATION FERRET". This should give you infinite lives/energy, but if it doesn't work, try typing it with a full stop at the end. Also, now pressing '0' to '9' allows you to skip levels.
You could also try pressing the 'HELP' key once the game has loaded for a secret message and now type in "WHAT A BUMMER" for infinite credits and the level skip.