Unlock Several Characters:
Beat all the levels (the different city sections ie: Shibuya-cho), including Golden Rhino levels, with a Jet ranking.

Love Shockers Gang:
Beat all Shibuya-cho levels with a "Jet" rank to unlock the Love Shockers gang.

Noise Tank Gang:
Beat all Benten-cho levels with a "Jet" rank to unlock the Noise Tank gang.

Poison Jam Gang:
Beat all Kogane-cho levels with a "Jet" rank to unlock the Poison Jam gang.

Goji Rokkaku Gang:
Unlock the Love Shockers, Noise Tank, and Poison Jam gangs, then Beat all the Grind City levels with a "Jet" rank to unlock the Goji Rokkaku gang.

Unlock Potts The Dog:
Finish the game once, and then proceed to unlock Poison Jam, Love Shockers, Noise Tank, and Goji. Then play again, completing Monster of Kogane before Benten Boogie. Following chapter two, you will be challenged by Noise Tank. You will unlock the happiest pup, Potts, by beating Noise Tank.

Hints

Jump Further:
When skating hold the R trigger then jump while still holding. You get x-tra distance and sometimes pull of a trick like a backflip.

Speed Burst:
To get a faster start, press forward and the R trigger at the same time to get a speed burst.

Chasing Rivals:
When racing a rival(Yo-Yo) for the first time just follow them to see where to go, where the shortcuts are, where the spraypaint is, and the place to graffiti.

Thanks to Revolution reader Revolver G-Wok, Richard Roodbeen, Mickey Braun and jojo(mark)!