Mitsume ga Tooru

At the title screen, press and hold A & B on the 2nd controller and Select on the 1st Controller, then press Start. If done right, the debug menu will appear.

Whenever you see the intro screen to a stage, the screen declaring the stage number, hold Select and then rapidly and alternatingly press Left and Right (or Up and Down) on the D-Pad. If you press them fast enough (usually eight times per second), every 8th press will net you an extra 1,000 money; you can easily rack up to 4,000 money per stage screen this way.

Whenever an enemy drops a coin, shoot at it before you take it. Four shots increase the value of a coin. Most coins' values can be increased two times beforte they disappear, but the '500' coins' values can be increased only once, because '1000' is the maximum value of a coin.