Diddy Kong Racing/Secrets

Car

 * Tapping instead of holding it during the entire course of the race will significantly increase your speed.
 * Release before hitting a zipper and don't accelerate until the green fire disappears. Smoke will appear in Adventure mode.

Hover

 * Rapidly tapping and  while steering will significantly increase your speed.
 * Hop while making sharp turns to avoid the extreme lag involved with turning in a Hovercraft.

Plane

 * Barrel Rolls increase speed by an enormous amount, but restrict your ability to turn. Use them on straight ways, and they are sometimes a better option than hitting a zipper. Be careful not to do a Somersault, because that will stop you in place for a second or two.
 * Release before hitting a zipper and don't accelerate until the green fire disappears. Smoke will appear in Adventure mode.