Instructions
Wait until the number 2 begins to disappear at the start of the game and hold down the gas -- "2" button with the Wii wheel and "A" button with the nun chuck. Make sure you do this while the 2 still is visible but fading.
Go to Mii Plaza and change the weight of your driver. This will affect play and provide new cart selections.
Hold down the gas -- "2" button with the Wii wheel and "A" button with the nunchuck -- as soon as you hit the ground after falling into something, like a pit or water. You will get moving faster when you get back on the track.
Do a trick like a wheelie to create a shock wave when you see the POW block on your screen and its ready to hit you. You will become immune to the POW block and won't spin out, although you will lose any items you have. You can do this on bikes and carts.