Quick start:
Press A to accelerate at the instant between the dimming of the second light and the flashing of the blue light at the start of the race. Performed correctly, your kart will rapidly accelerate ahead of the other racers. Note that accelerating too soon will result in a spin out.
Hold A for the first two lights at the start of the race. Release the button when the second light dims. Press A at the start of the third light as if you were getting the boost start. This should make you spin out while getting a boost start.

When lined up for a race, repeatedly tap A until it "Go" appears. You will start at full speed immediately.

Quick restart:
After falling off a course, watch as Lukita places your kart back on the track. Press A when your kart is two or three tire widths from the track to immediately resume the race.
When something hits you, press A three times, but hold the button on the third time. If done correctly, your Kart will quickly boost up to its maximum speed.

Item select:
This trick is very useful in battle mode, versus mode, and grand prix. While controlling your character, drive into an item box and immediately pause the game as soon as the items start to change in the box. The roulette action of the items will stop. Resume the game and immediately press Z or C-Down. The box will usually stop on the item after the item that was displayed on the pause screen. Keep spinning or, if you are lucky, land on the item that was on the pause screen. Just take some time to study the order in which the items appear to select the desired item.

Spiked Shell locations:
There following two places have hidden item boxes that will always give you Spiked Shells if you wait until its done going through items. You can find them in the Mushroom Cup on Luigi Raceway and Koopa Troopa Beach.
In Luigi Raceway, get to the item box hanging on the hot-air balloon that hovers up and down a short distance after the starting line. To reach it, you must either have perfect timing when racing on the course or you can wait on the shadow location on the track until it drops on you.
On Koopa Troopa Beach, race through most of the course until you see a long, narrow ramp around a corner. You must have a Mushroom power-up for this to work. Drive up the ramp at top speed and steer slightly to the right. Use a Mushroom near the top to fly in midair over the giant turtle rock and hit the item box. Wait for it to stop to get a Spiked Shell.

Kalimari Desert:
Recommended kart:
The best racer for Kalimari Desert is Yoshi because he has good handling, is moderately fast, and can brake quite well.

Koopa Troopa Beach:
Faster driving:
When you are racing in this circuit, it is best to drive your car where the sand ends and the water beings. The water is shallow and always is drifting in and out. This will give you better speed, as well as will allow you to steer much better.

Koopa Troopa Beach:
Always get a spiked shell:
Go up the giant ramp near the end. You will always get a spiked shell.