When the title screen appears, hold down A, B and Select.
When at the title screen, press A+Select, B+Select, A+Select then press Start (you should hear a sound letting you know it was done correctly) and during gameplay, hold down Select and press the corresponding direction on the D-Pad below:
Up-Right Extra Lives
Up Jumping Shoes
Up-Left Moon Mirror
Down-Left Weapon Strength
Down-Right Zap's Energy
In Stage 2, after the mid-point (of where you fight the dog and the soldier at the stairway), go to the area where Kim's second shop is. Continue right and find some pits. Jump down the second pit, and you'll find yourself in a hidden area, of where you'll battle a warrior guarding a cage. After a few hits, the warrior will retreat. A princess will appear. She will give you the Moon Mirror.