Infinite Lives

ign.com:

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.

Infinite Lives on World 4-5

ign.com:

To get infinite 1-UPs, you needn’t go any further than World 4-5. There, head to the first pipe spawning Bobombs. Clear a way to the lower (indestructible) floor, but leave the left and right edges in tact. Grab a Koopa Shell and toss it into the pitt, and then position yourself far away enough from the pipe so the Bobombs continue to respawn. If done correctly, the Koopa shell will bounce left and right endlessly, racking you up 99 lives in no time.

Infinite Lives

about.com:

In World 2-4, just past the two Piranha Plants (after the mid-level point) you will come to a staircase with a turtle coming down it. Jump on the turtle just as it is about to go off the third stair (third from the bottom). If done correctly, Mario will continue to jump on the turtle and earn up to 99 lives. If you miss the turtle, go back towards the left and return to the staircase and the turtle will respawn. This is a reference to the original Super Mario Bros., in which you could do the same trick to get unlimited lives.