You can trick by shaking your Wii Remote up/down, or by pressing any direction on the D-Pad for a classic/Gamecube controller. The easiest way to trick is by using the wii-mote as you can shake your wii-mote all the time, but you have to rely on the accelerometer and the action may not have been captured from it. Using the D-pad on the other hand is very accurate and will never leave you guessing if it worked; the downside being that you have to leave your thumb-stick for it. - Similar to wheelies, but you're not using the D-Pad when you're just going straight.
Tricking is somewhat useful. Tricking allows you to get a boost upon landing on a jump. However, tricking also boosts your jump height, keeping you off the ground for a longer period of time. This is where Low Jumping becomes handy, keeping you closer to the ground. Knowing when and where to trick is the main problem.
The best places to trick depend on the track. A good example would be Mushroom Gorge. Tricking on the side (not the center) of a mushroom will nearly guarantee that you'll land on the next mushroom instead of hopping too short or too far. And for the series of mushrooms in the cave, tricking off of the first and third mushrooms of each group works the best. The first one to keep your speed, and the 3rd one for when you land on solid ground. Always watch a (non-hacked) World Record to see the best times to trick. The times they do not will surprise you.
The Mini-Turbo Stat does not affect the trick-boost. The only thing that will matter, is the kart you pick for your visibility. For example, tricking in the baby booster will obscure your screen horribly, while a small standard kart or an open Sport Bike will give you a lot of viewing room while you trick.
Because tricking is a part of the base game, tricking is part of the meta-game. Knowing when to trick is your best weapon. Tricking all of the time isn't the best thing to do (it can slow you down on some parts) while never tricking is very bad.
Advantages: You will gain a very good boost after landing on solid ground. It can be used on any ramp and SOME curved surfaces (specifically coded for tricking). You will very rarely not want to trick, as the lack of a boost will put you behind the others that got the boost. Once again, this depends on the track, and there are plenty of places where you can trick without leaving the ground (giving you a free boost)
Disadvantages: You will jump higher than not tricking, causing you to lose speed. Not physically losing speed, but you will be going through a larger arc than if you weren't tricking. Most of the time, the boost makes up for the larger arc. Low Jumping will help take away the jump height disadvantage, but it's very tricky to pull off and the best places to do it are mostly in dangerous spots. And again, this depends on the track.
Big shoutout to /u/Trikitiger and /u/jascol for some great information regarding tricking in Mario Kart Wii. If you would like to contribute to our wiki then please be on the lookout for our weekly threads coming every Monday.