Controller
Mario Kart Wii allows players to use the Wii motion controller along with a plastic wheel-shaped mount or "Wii Wheel" to control their kart. While the Wii Wheel can be fun, most of the best players use a standard Gamecube controller to play Mario Kart Wii. The level of control on a gamepad is usually tighter and easier once you learn how to use it; if you really want to get better at the game, you should consider using a Gamecube controller.
Manual vs. Automatic
When you select your character and vehicle in Mario Kart Wii, you have an option of manual or automatic steering. Automatic steering is often easier for beginners and will allow you to corner more sharply by simply steering your vehicle one way or the other. Manual is the preferred setting for high-level racers, since it allows you to hop by pressing the "R" button on Gamecube controllers and use skid turns by holding the button through a turn, resulting in a boost of speed or "mini nitro" when you let go of the button.
Karts vs. Bikes
Despite the title of the game, karts are generally inferior to bikes. The three best vehicles in the game for racing online are the Bowser bike/flame runner, mach bike and the bullet bike. The reason bikes are superior to karts is that they have the ability to do a wheelie by pressing up on the directional pad with a Gamecube controller. A wheelie allows the bike to go faster than normal in a straight line. The drawback of using wheelies is that if another vehicle bumps you as you do a wheelie, your bike will wobble and lose a significant amount of speed.
Item Usage
One of the most important parts of Mario Kart strategy is proper item usage. In most games, items ultimately determine the winner because they have many powerful effects that can easily change fortunes for the better or worse. The better your place, the lesser the items you will receive, so when you are in first or second, you won't get many good items, while you almost always get good items in last place. In first place, your goal with items should be to protect yourself and lay traps for racers behind you. To do this, always hold back on the control stick and drag a shell or banana peel behind you to prevent any fired shells from hitting you. If you have an extra item in your inventory, wait until there is a narrow spot in the road then lay the item there to hamper racers behind you.
When you are doing poorly, it is imperative that you get items. Even coming to a complete stop and going in reverse to get an item box can end up helping you get ahead. In general, you should use bullet bill or lightning as soon as you get them, but if you get mushrooms, save them and use them to pass through areas of rough terrain quickly. If you manage to get into first place while still having a mushroom in your inventory, save it for a shortcut or to start up quickly if you get slowed down by something. You can also use a mushroom item to avoid a blue shell while in first place if you use it the instant before the blue shell drops on your head and explodes.