While riding on a Yoshi, jump on the bullets seven or eight times to get extra lives.
Kill eight enemies while using a star to get an extra life. You can keep it going until the star runs out if you are fast enough.
Kill eight enemies with a turtle shell to get an extra life. You can keep hitting the turtle shell until there are no more enemies if you run fast enough.
Jump on seven turtles to get an extra life. You can keep jumping on the turtles until there are no more to jump on if you are fast enough.
Freeze the Pokeies, then do a quick Ground Pound to get at least five extra lives.