Win three races in championship mode to get a new girlfriend.
To get bigger air, hold L as you go up the ramp. Release L (ideally when the gauge is full) at the lip of the ramp.
Since the back flip trick goes very fast and looks unrealistic, there is another and much better looking way to do one. Use the "Bigger air" trick. After you launch off the ramp, hold Back and you will do a better looking and more realistic back flip. You can also do a front flip like this, but it requires some skill, and big air.