Keep in mind that speed isn't everything in this game.
You'll find it much easier to make a stable landing if you always try to land your wheel entirely parallel with whatever landscape is below.
Good trick to do is rocking your rider backwards and forwards to gently lift it over the challenge obsticle.
Each of the Challenge levels of the game has a special trick up its sleeve.
To access the community's growing pile of user-created levels, head to the Online section from the main menu and look at what's available.