I used this on the version of the game that came hard-coded into the 2nd model of Master System. It works on that version at least.

When you get the Game over screen (which happens way too often to make the game any fun...) you can continue if you hold up and hammer the 2 button about a dozen times before the game actually resets. It costs $400 to do.

If you lose all your lives, have over $800, and wish to continue, hold Up on the D-Pad, and press Button 2 8 times.

This is how to beat the Janken bosses when you play against them, without having to fight them if you lose.

1st: Choose Rock in the first game, then Scissors in the second
2nd: Scissors, then Paper
3rd: Rock, Scissors
4th: Paper, Paper
5th: Rock, Rock
6th: Rock, Scissors
7th: Paper, Paper

(Must be done in that order from the start of the game, if you choose one different along the way all the rest will be different)