Seventy-five lives:
Hold L + Right + Y + Select and press Start at the title screen. Continue holding Start until the text "Zippity Splat" appears on top of the screen.

Ninety-nine lives:
Press Select at the title screen to display the options screen. Then, press A, Y, A, Y, X, Y, B, A, R, L.

Hint: Zippity Splat: Extra life:
After you have passed the blue bridge, jump up on the brown object that carries you up the mountain. When you almost get to the top, run off the right side and press Right. If you successfully make it into the mountain, go down the steps, then go straight until you cannot go straight any further. Jump up just when you get wall, and go straight through. Run until you see a small brown box and use your beak to open the box to get an extra life.