Get a cape and find a bunch of Wigglers close together. Hold Y and B as you jump on them one after the other without touching the ground. As usual, you will get more and more points for each one you stomp, then eventually 1UP, 2UP, 3UP. Each time after 3UP you will get a bunch of weird symbols, some coins, and a lot of points. For example, while experimenting with this glitch, we went from 6,714,540 to 7,247,020 to 7,799,980 to 7,912,620 points with each jump.

Ideal course: Forest of Illusion 1. A bit after the Midway Gate, there are three Wigglers walking on floating log platforms. Keep scrolling back and forth so that the Wigglers respawn and you can stomp them again and again. (Note: It's OK to jump on the Koopas here. They'll still give normal 1-Ups and won't disrupt the glitch.)