This is easiet when you are blue shell Mario. In World 2-2, go past the three turtles, then past the cluster of power-ups, to find four coins enclosed by a cage of blocks.

When you take those out, you create a small, walled up gap where if you throw a shell into it, it will bounce back and forth forever. You would already have been attacked by a creature in a cloud throwing spiked shells at you. Stand close to it without stepping on the shell, and let the cloud creature keep tossing the spikeys into the gap, where the shell bouncing back and forth will take them out.

Press down (crouch) to hide in your blue shell so you don't get hurt. Soon, every spikey that's thrown into the gap and taken out by the bouncing shell will give you one more life. You'll reach 99 lives in no time.

In principle this works wherever you can get a shell to bounce back and forth perpetually, and there's an endless supply of enemies dropping in to be taken out.