Boost item
Right after you are hit with an item, tap the "A" button 3 times really fast, you will start going full speed again.

Boost Start
When you are starting a race, just before the light turns blue, hold the gas button (A). Your kart will go faster than normal, like you are using a mushroom.

Wario Stadium Shortcut 1
When you start the level, go full speed on the first bump and press (R) right when you hit the wall. If you time it right, you will go on the other side and be halfway through the course.