At the LucasArts logo at the beginning of the game, move the joystick up and then press fire, then down and fire, left and fire, and right and fire. If done correctly, you'll hear a phone ringing and a voice answering "LucasArts" Once activated, you can skip to any level by pressing:
1-9 Goes to the respective level
a-h Goes to the rest of the chapters (a goes to 10; b goes to 11, etc.)
You can also repair your ship by pressing '+' on the keypad or skip to the next level by pressing Esc...