Work your way through Course 1 in Mushroom World, until you reach the stairs near the first star coin. Stand on the second or third step and wait for the descending enemy to reach you. Jump lightly to land on it and send its shell toward the wall. The shell will bounce back and Mario will continue to jump on it, racking up points and eventually earning you extra lives. This should be a familiar trick if you played the original Super Mario Bros. on NES. It won't work in New Super Mario Bros. 2 if you try to perform the trick as Golden Mario, though (your enemy's shell will eventually disappear instead of continuing to grant extra lives).