Rocket Start
Although you won't find it any manual, every Mario Kart game since Mario Kart 64 has contained a hidden "rocket start" feature. This feature gives your character a mushroom boost at the beginning of the race. To enable the rocket start in Mario Kart Wii, hold the accelerate button after the "2" disappears. Once the race starts, your car will take off from the start line just like a rocket.
Ramp Stunts
When you see a ramp, go after it at full speed. When your character is at its highest point mid-air, press the "trick" button. Once you land successfully back on the ground, you will have an automatic speed boost that will last a few seconds. If you jump on multiple ramps, you can chain together these boosts for a longer speed run. Note: if you fail a trick, you character will fall and you will experience a speed penalty.
Recovery Boost
If you fall off the track at any point in the race, you will be picked up by the friendly Lakitu. However, Lakitu picks your character up painfully slow, and one fall can seriously hamper your chances of winning a race. However, if you press and hold the accelerate button as soon as your vehicle's wheel touches the ground, you will get an instant speed boost. Note: take care not to press the acceleration button too early, as your kart will stall if you don't time your button press perfectly.