Unlock Cheats Menu
If you have v1.4 go to the 'Main' menu and tap the corners of the screen in the following order; bottom-right, top-left, top-right, bottom-left to display the Cheat menu.
To gain a lot of height when launching off a ramp, activate jet propulsion as you ascend the ramp. That serves to kick your car into gear as it prepares to go off the ramp, giving it the speed boost necessary for higher altitudes.
Be careful in applying the brakes. It's usually much wiser to initiate deceleration using the braking flaps since the regular brakes tend to make your car fishtail.
You don't need to complete all of the stages in a given level of difficulty to unlock the next one; only three time trials and four platforming stages have to be finished to unlock the next set.