Hold T + A + B for 99 rounds of ammunition and an extra life.
Hold C + T + [Space] to receive the pogo stick, all key cards, and ray gun charges. Alternatively, press [Ctrl] + [Tab] + [Space].
Hold G + O + D. This also allows better jumps.
Press [Shift] + [Tab] while at the map level.
Pogo bouncing in air:
Enable the "Invincibility" code, then take out your pogo stick and hold [Ctrl] to go to the ceiling. Release and you can keep pressing it to stay in the air. This works great for things such as teddy bears.