Shortcuts
Go to the left instead of taking the first turn in the Mushroom Gorge race; this make you bounce on two mushrooms and over the fence. Follow the mine carts near the end of the lap in Wario's Gold Mine and you'll land in a secret path with some speed-ups. Avoid the carts while following the path. Stay to your right at the start of the Maple Treeway race. Look for the entrance to the shortcut after a wooden fence and a big tree root; this will take you to the front of the cannon. On the Coconut Mall race, stay on the same level after the first set of escalators and go right to the corner. Cut into the shop and use a mushroom when the carpet slows you down; this will take you out above the plaza area after the second set of escalators.
Tips
It's best to choose manual steering unless you're using the Wii Wheel to avoid drift; go with automatic if you have the wheel. Press and hold the button to accelerate at the start of a race as the hand points at 11 o'clock when the countdown gets to "2" to get a rocket start. Avoid hitting walls if at all possible, as this brings your driving to a crawl. Hold the "A" and "B" buttons together if you need to turn around quickly. Flick up on the Wiimote or tap "Up" on the D-pad while launching from a ramp to perform a trick; this also gives you a boost when you land.
Items
You can throw collected items ahead of or behind you by pressing up or down on the directional pad, respectively. Drag items behind you by holding down the button you use to release an item; this will let you block attacks coming at you and time your item drops more precisely. This is especially recommend if you're in first place and are expecting attacks.