When you have a single shell or banana, you can hold ZL to keep it behind your kart as a defense, although they won't shield you from strikes to the side. You can also evade attacks by taking sharp turns, jumping, and putting other players or characters in between you and the pursuing item.
Coins increase your speed, so collect as many as you can. You will drop some if you are hit, and if you hit other players with items, they will drop coins you can collect.
Move the analog stick or D-pad left/right as you turn to drift and build up a small boost.
To get a boost at the start of the race, press and hold the accelerator button when you hear the second beep in the countdown.
Track Boosts and Shortcuts
Even when tracks have boosts along the outer edge, it's often quicker to hug the interior of the curve. Many tracks also have shortcuts you can take.
Some items lock on to players ahead of you, but they won't lock on to players behind you if you shoot them behind.
If you get three shells, don't throw them all at the same racer. If you hit the racer with your first shell, try to get ahead of them so you can use your next shell on someone else.