ok lately people been asking how to do most of the basic and some advanced techniques in MKW, so i am hoping this guide will help answer those questions that any players of the game or anyone new to it may have.
UPDATED 09/07/2008 - made the character chart neater and made a legend for the chart, added extra info for rosalina in character section, added snaking to advanced techniques
First i will start with the Boosts (will not bother with the control setup)
Boosts
Mini Boost at start
Hold Gas when the countdowns start to fade at 2. The amount of speed and length of the boost depends how long you held the gas for when the 2 fades. perfect this and you will always have an advantage against new players or players that don't know how.
Mini Boost when stopped
when stopped completely by any type of shell or other item and you have a bike character who has horrible acceleration. if you hold gas and brake at the same time for i think 3 seconds it will show the blue sparks at the back of your vehicle and if you let go of brake you will get the boost to put you back at full speed.
Mini Boost from falling
when you fall into the old faithful yet annoying water/lava you will be brought out by Lakitu (cloud guy) which horribly stops you. when your dropped by it, if you hold gas as soon as you touch the ground you will get a mini boost right away and start moving at full speed.
Mini Boost from ramps
when going up a ramp whether it has the speed boost or not, you can get a speed boost if you do a trick off of it. it is not as powerful but you will get it once you hit the ground.
Mini Boost from drift (manual only)
when coming up to a turn, if you start turning while holding the drift button you will start to build up a blue spark, let go of drift to get the basic Mini boost, but if you are using a kart if you continue to drift you will build up a orange spark which is a even stronger Boost but is only for karts only
Karts/Bikes/Characters
weight classes
There are 3 different types of weight classes.
light - top 2 rows
medium - middle 2 rows
heavy - bottom 2 rows
they determine what set of karts/bikes you will have to pick from.
the 2 in the bottom right which allow you to race as your Mii. there weight class is determined by how big or small you made your Mii character.
Stats
the stats determine how your vehicle will run
Speed - detemines your top speed (effects how fast you will move at top
speed)
Weight - determines how heavy your vehicle (effects jumps and ramming
of lighter player's karts
Acceleration - determines how fast you reach your top speed. (effects
how fast you recover from being hit by items)
Handling - detemines how well you turn (effects how you turn on tight
corners)
drift (manual only) - determines how fast you obtain the mini boost from
drifting (effects how fast you obtain it and how
long it lasts)
Offroad - determines how well you move off track (effects how you move
in grass,water,dirt,ice,ect)
Mini Boost - determine how effective mini boost are (effects how strong
and the length of the mini boost)
Characters
each character has there own stats that effect the vehicle by a little bit which can also effect the game completely when it comes to winning
this is the only part of the guide that was copied and paste and now edited by me to make it easier for everyone to read
LEGEND:
S: speed
W: Weight
A: Acceleration
H: Handling
D: Drift
O: Offroad
M: Mini Boost
TT: TimeTrials
SR: Star Rank
Character ------- | ---Unlock-- | --- Class ---| S | W | A | H | D | O | M
-------------+--------------------+--------+---+---+---+---+---+---+---
Baby Mario -------
| ------------
| ---Light -----
| 0 | 8 | 0 | 6 | 0 | 0 | 0
Baby Luigi --------
|8 TT Ghosts
|---- Light-----
| 5 | 8 | 0 | 0 | 0 | 0 | 0
Baby Peach ------
| -----------
|--- Light -----
| 3 | 6 | 3 | 3 | 0 | 0 | 0
Baby Daisy -------
|50 cc new SR
|----Light-----
| 5 | 6 | 0 | 0 | 0 | 0 | 3
-------------+--------------------+--------+---+---+---+---+---+---+---
Toad -------------
| -----------
| --- Light ----
| 0 | 0 | 6 | 0 | 6 | 0 | 0
Toadette ---------
| --32 TT----
| --- Light ----
| 3 | 0 | 0 | 0 | 0 | 6 | 0
Koopa Troopa -----
| -----------
| ---- Light ---
| 0 | 0 | 0 | 3 | 0 | 0 | 6
Dry Bones --------
| 100cc Leaf-
| ---- Light ---
| 0 | 0 | 3 | 0 | 3 | 0 | 6
-------------+--------------------+--------+---+---+---+---+---+---+---
Mii (Light) --------
|100cc Special
| ---- Light ---
| 3 | 3 | 0 | 0 | 3 | 0 | 3
-------------+--------------------+--------+---+---+---+---+---+---+---
Mario -------------
| -----------
| --- Medium --
| 0 | 6 | 2 | 2 | 3 | 0 | 0
Luigi --------------
| -----------
| --- Medium --
| 2 | 6 | 0 | 0 | 0 | 0 | 0
Peach ------------
| ------------
| -- Medium --
| 2 | 0 | 5 | 0 | 6 | 0 | 0
Daisy -------------
|150cc Special
| -- Medium --
| 4 | 0 | 0 | 2 | 0 | 0 | 3
-------------+--------------------+--------+---+---+---+---+---+---+---
Yoshi -------------
| ------------
| -- Medium --
| 0 | 3 | 0 | 0 | 3 | 5 | 0
Birdo -------------
| -beat 16 TT-
| -- Medium --
| 0 | 3 | 0 | 0 | 0 | 3 | 5
Diddy Kong --------
|50cc Lightning
| -- Medium -
| 0 | 0 | 3 | 0 | 3 | 0 | 5
Bowser Jr. --------
|100cc Retro SR
| -- Medium -
| 0 | 0 | 0 | 0 | 0 | 3 | 3
-------------+--------------------+--------+---+---+---+---+---+---+---
Mii (Medium) ------
| 100cc Special
| -- Medium -
| 3 | 3 | 0 | 0 | 0 | 3 | 3
-------------+--------------------+--------+---+---+---+---+---+---+---
Wario -------------
| -------------
| -- Heavy --
| 0 | 3 | 0 | 0 | 0 | 3 | 6
Waluigi ------------
| -------------
| -- Heavy --
| 0 | 0 | 6 | 0 | 5 | 3 | 0
Donkey Kong ------
| -------------
| -- Heavy --
| 0 | 3 | 2 | 2 | 0 | 0 | 3
Bowser ------------
| ------------- | -- Heavy --
| 2 | 5 | 0 | 0 | 3 | 0 | 0
-------------+--------------------+--------+---+---+---+---+---+---+---
King Boo -----------
| - 50cc Star -
| -- Heavy --
| 0 | 0 | 0 | 5 | 0 | 3 | 0
Rosalina -----------
| - Mirror All SR
| -- Heavy --
| 3 | 0 | 0 | 3 | 0 | 0 | 3
Funky Kong --------
| - 4 TT Ghosts
| -- Heavy --
| 4 | 0 | 0 | 0 | 0 | 3 | 0
Dry Bowser ---------
|150cc New SR
| -- Heavy --
| 0 | 0 | 0 | 0 | 0 | 6 | 6
-------------+--------------------+--------+---+---+---+---+---+---+---
Mii (Heavy) ---------
| 100cc Special
| -- Heavy --
| 3 | 0 | 3 | 3 | 3 | 0 | 0
ROSALINA
there are two ways to unlock her with one being get a star rank in all cups on mirror mode. the other way is if you have a super mario galaxys saved data on your Wii it will unlock her as soon as you choose your license
Tricks and their uses
Tricks
there are many uses that can either help you avoid certain items or give you a little boost. Tricks range from doing tricks off of ramps, avoiding a certain annoying pow block, or using wheelies for a little extra speed
control set up (because you guys asked)
this is how you do tricks for each of the controllers depending on which you use.
Wheel - to do a trick with a wheel all you have to do is push the wheel
away from you in a quick motion
Wiimote - (same as wheel just without the wheel :P)
Wiimote + Nunchuk - shake the wii remote
Gamecube Controller - just press any of the D-pad buttons --> +
Classic Controller - (same as the Gamecube controller)
Ramps
doing a trick as soon as you launch off anything that sends you flying upwards whether its a little bump or one of the boost ramps will send you flying a little higher while you watch your character do a trick and once it touches the ground it will get a little speed boost
Wheelie (bikes only)
when on a bike if you do a trick while on the ground (D-pad users = up or down) you will ride into a wheelie which increases your speed. warning when doing a wheelie make sure your on a straight path, if you have to do a sudden turn do a trick again to stop the wheelie because you cannot turn while wheeling. also i noticed if you try turning whether its a small turn you will lose speed, you can only be at your fastest when going straight.
Pow Block
the dreaded pow block which always hits everyone in front of you...
the only way to escape it fully would be if your in the air, but even so you can still a avoid the speed drop if you do a trick as it hit you. you will still lose your item but you won't lose any speed.
Advanced techniques
Blue shell escape
there are a couple of ways i know of how to escape a blue shell thats homing in after you but requires alot of luck.
one way is by using a mushroom just before it hits you. if done correctly you will be boosted out of the explosion unharmed
The other way i know is when its about to hit you, you ride into one of those huge boost ramps (example - rainbow road's star boost) what happens is it breaks the blue shell upon impact.
Snaking (as requested by Ownage)
personally i never like snaking because became to cheap in mario kart DS to push you pass your speed limit. MKW made it so you could not snake in the game. You can snake but its not as good as the MKDS and 64 version but because it was requested i will explain how to do it
basically to snake is to continually be mini boosting through a straight lane by continually drifitng left and right (basically like a snake slithering). Its best to start Snaking as soon as you enter a straight lane from a turn so you can continue to drift left then let go of the drift as soon as you get the blue spark then right away drift right till you get blue spark and so on.
IF used properly these can in fact be a big different in any match on mario kart wii
if you like this guide rep me (or not i really don't care about rep :P)
Thanks Mods for sticking this 
if you guys want to add anything else to the guide just tell me