Mario Kart DS (NDS)

Time Trial passwords
In the Records section, select a time trial record and press Up, Down, L, R, Y, A. This makes a 16-character code appear that might have been planned for use in tournaments.
Disappearing ink
When your screen is gooped up with Blooper ink, run over a dash panel or use a Mushroom item to quickly clear your screen.
Check before you wreck
Checking the map on the bottom screen is an easy way to spot a fake item box on the track. On the close-up (default) map view, fake item boxes show up as bright red squares.
Alternate startup sound
Mario has a different greeting depending on which Nintendo DS you're using. Right as the game loads, Mario says "Yahoo!" on the original DS, but on the DS Lite he says "Here we go!" instead.
Speed boost
For a burst of speed to start the race, press and hold the A button right after the second beep during the countdown.
Lakitu boost
To quickly get back up to speed after falling off the course, press and hold the A button right as Lakitu places your kart back on the track. If your timing is true, you'll get a short burst of speed.