This only works on 100cc and 150 cc. When you hear one coming, get into a powerslide with red sparks. Hold it while the shell is circleing (spelling), and when it goes quiet, release the slide and jump. It takes a lot of practice, and you can only dodge them in 100 cc sometimes. Just make sure you jump. Also, if you have a mushroom, right before the shell hits, boost and jump. In both cases, the shell should hit right behind you.