Collect 50 rings, then enter the special stage and get an emerald. Let yourself die, then collect another 50 rings and re-enter the special stage.

Give your rings to the same person you used when you went to the special stage the first time. You will go to the same stage where you found the emerald, but if you complete it this time, you'll get an extra life.

Collect 100 rings to get a second extra life. Die and repeat the cycle as often as desired.