Pick up enough speed and go up a ramp (Halfpipe e.t.c) Then do a backflip LT+down. You should go into slow motion.While in the air Jump off your board, LB+RB the do a manual you still should be in slow motion. If you are in manual while in slow motion,pull of as many tricks as you can.
There are alot of missions that require you to do tricks in Nail The Trick mode. These are the best times to train. You can learn how to use both sticks at one time and how to time new tricks. There are a few to do. One of the best spots to train is in the City Center (the place with the general and the big government buildings). You start on top of a building and have to jump off of an air vent that shoots you higher.
Also, you can go to the Fun Park and launch yourself at the RollerCoaster. This is pretty high, too, but the first one is the best.