Coins If you perform tricks that have plenty of air-time you will be get more coins at the end of each run when your score is tallied up. Try not to endanger your run though by trying to perform an extravagant trick that makes you crash land.
Cash If you have real-world money to spend you can go to the cash store to purchase ugrades if you don't want to spend time grinding levels for coins.
Upgrades When you have gathered enough coins you will be able to spend them on upgrades for your car. These also include new landscapes to race through and new cars to do the racing in. Unless you are experiencing problems in a certain area it is best to upgrade each of the four engine systems equally instead of focusing on one in particular.
Steep Inclines If you are struggling to get up very steep inclines you need to think about investing in your engine, it also helps to upgrade your tires as this will help your traction.
Tires Upgrading your tires will improve your speed as well as traction.
Handling You can improve your handling so it makes a big difference by ugrading the suspension and 4WD. An improved suspension will also help you to navigate smaller, bumpier patches in the ground without flipping over.
Throttle Although you will not earn much air-time you will survive longer if you pump the gas gently to just about clear steep climbs so your car settles gently into the decent. You will also be able to gather more coins playing cautiously.
Clear Bridges Without Flipping Over This can be prevented by not leaving the bridge at full speed, if you instead brake well in advance of clearing the last plank you should clear the bouncing bridge with grace and some speed. Don't brake too sharply though otherwise you will still flip over.
Fuel Meter Keep an eye out for the red fuel canisters that occasionally appear on the road and pick them up as they will replenish the fuel meter because if you allow it to expire your high-score attempt will end. Land quickly if necessary to take advantage of a fuel top-up.