Bonus life:
Stand in a moat and kill seven ninjas with your character's sword. Catch the blue creature that appears to receive a bonus life.
Ten lives:
Hold Select + Start + A + B on controller one and A + B on controller two. Power on the NES while those buttons are held. Release the buttons when the title screen appears. Then, hold A + B on controller one and B on controller two. While keeping those buttons held, press Start on controller one.