If you need to get down to the ground quickly to gather up some coins or put fuel in the tank you need to hit both of the buttons simultaneously, when you do this your car will plummet to the ground.
Free Crates Easily
At the title screen, press "Free Crates" and wait for the video to stream. When it's fully streamed and playing, skip to the end of the video and the game will think you watched the whole thing. Repeat this for as many cates as desired.
Free Fuel
At the start of every trip you are given a limited amount of fuel in your tank but more gasoline can be added to it by picking up the red fuel canisters which are littered along the road and by trying to balance your massive jumps with slamming down so you get more gas quickly.
More Coins
At the 'Main' menu press the button that is marked 'Free Crate' and watch the adverts to pick up more coins. You can keep doing this until you get the desired amont of coins. You can also connect with friends via Facebook to get coins.