in the level downtown go to the bowling alley and go inside next to the ped. Once you're there go to the far left ball return and stall on it for 40 sec. If you do this correctly you should get about 1000 stokens.(hint this cheat only works sometimes and make sure to use the infinite grind cheat:balancegalore.)
In the downtown level,there is a way to get hundreds of stokens with little effort. Wait at the toll booth until a car comes, and stall on top of it (jump and press triangle+R2). The car should circle around town, and if you can keep your balance until it gets back to the toll booth, (it's easy using cheat codes) you will be rich. Just don't let the car take you out of bounds. (Of course, you could just cheat code the skateshop...) Anyway, I'll explain how this works. Everyone in town will get to see your stall combo and add their stokens. (Try doing tricks during your stall by double pressing face buttons)
This glitch is real cool. In the fun park, go to the pirate area and get off your board, then stand on top of the skull where the waterfall is, then rotate the camera angle so you are looking at your skater from behind the waterfall. If it is angled right, your skater will have no hair!
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.