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Skating is all about moves. Start with these.
Shove it - 250 points
Impossible -250 points
Kickflip- 100 points
Heelflip - 100 points
Then you can start doing things like Wall Rides, Nose Grinds, and Mega Ollies.
Once you get all 30 tapes, you can play in career mode as Officer Dick apparently, but I've never gotten that far.