Play the game for 30 hours total to unlock all skins for all cars.
Select where you want to go, then choose the track. After you do that, turn Xbox rules off and proceed. After everybody gets their cars, go back through the process. However, instead of pressing A to continue after you change Xbox Rules back on, press B all the way back to the lobby. Then, start the race, but make sure that their car does not change. This works about 99% of the time.
While driving, click the Right Analog-stick.
This trick allows to use a car from one class (for example, Hill Climb) while racing in a different class. It can only be done online, and you must be the host of the game. Start your game with the game type that you want to race in (for example, Rally). Then, go to the game settings screen and select the class of the car you wish to use (for example, Hill Climb). Then, press B to exit back to the lobby. Then, go to car selection screen and you may choose a Hill Climb car in a Rally race. Note: Hill Climbs are much faster.