Extra Lives
At the title screen, press B, A, Down, Up, Down, Up on Controller 2, then press Start on Controller 1.

Faster Fire Punch
As you see above it tells you to hold A button until you flash and release, but sometimes this takes much to long. Hit start, pause then hold A, count to five then release pause while still holding A, you will start flashing instantaniously.

Flaming Punch
Hold A until your fighter flashes, then release it.

Play as Super Man
At the title screen press Up, Down, Left, Right, A, B, A, B, B, A, B, A, Right, Left, Down, Up. You have to do it really fast, so expect it to take a while to master.

Super Spin Kick
Hit A + B + the direction you want to attack in (only Left or Right) simultaneously to unleash an Super Spin Kick which does double damage on an enemy. It takes some getting used to, but it is worth it.

Note: it does not clear the screen in the direction you are facing like the Flaming Punch. It only works if you hit the guys.