Love Shockers gang:
Successfully complete all Shibuya-cho levels with a Jet rank.
Noise Tank gang:
Successfully complete all Benten-cho levels with a Jet rank.
Poison Jam gang:
Successfully complete all Kogane-cho levels with a Jet rank.
Goji Rokkaku:
Successfully complete all the Grind City levels with a Jet rank.

Pots (the Dog):
First, unlock all the bonus characters (Noise Tanks,
Poison Jam, Love Shockers, and Goji). After completing the game,
play the "Monster of Kogane" level before "Benten Boogie". After completing those levels, a Noise Tank will
challenge you after Chapter 2. Defeat the Noise Tank Challenge
and you can play as Pots.

Skate backward:
Press Down, Up while skating.
Information in this section was contributed by MEWSMDBZ.
Select characters faster:
Hold R at the character selection screen to
cycle through the players faster.

Hint: Plague of frogs:
Start a new game. Complete Shibuya GG with a Jet ranking.
Then, complete Love Trap with a Jet ranking. Finally, defeat the
Love Attack with a Jet ranking. You should not be able to complete
any more Love Shockers levels. After this, complete Benten Boogie
with a Jet ranking. If done correctly, you should have a plague
of frogs in your garage.

Hint: Jet Technique trial:
Stay skating in the zone you start in.  For example, in Shabuyo-Cho
you start at the bus terminal.  The zone you start in is usually
the easiest one to get the most points.
Instead of just messing around, find a way you can make a circle
that you can just skate around in, getting points.  For example,
at the bus terminal, grind along the railings, board slide on
the billboards, and jump to each obstacle until you get back where
you started without falling.  From there, go around and around
repeatedly until you fall off.  You will get hundreds of points.
If you are in range of a graffiti location and you have at least
one can, spray away.  You will get 500 points just for that.
Do not stop skating.  You only have a limited amount of time.

Hint: Jet Graffiti trial:
Spray every single space, even the green arrows.
Use a character with a high graffiti gauge (Gum, Parana, or Yo-Yo).
 It will be harder, but you will get more points.

Hint: Jet Crush trial:
Do not bump into your rival.  You will get hurt and slow down.
 Get out of your rival's way -- if they slam into you, you will
the one that will get hurt.
Find ways of getting by your rival without getting hurt.  You
can jump or dash (both maneuvers if executed correctly can help
you out more than you would suspect).
Find shortcuts.  Some are obvious and some take a little more
experimentation For example: In Shabuya-cho while racing Beat,
before jumping of the bridge into the half pipe, get ahead by
grabbing onto a bus.
Do not stop.  One second could be the difference between a nitro
and a jet. Timing is everything.  Fail on one thing and you can
get very far behind.

Hint: Panties:
Stand atop any half pipe exept for the sewers in the
Benten-cho levels. Stop, then very slowly walk over the edge of
the halfpipe with any character that wears a skirt (for example,
Gum, Mew, etc.) while rapidly pressing L. The camera will
mysteriously zoom right up the characters skirt revealing their
panties for no apparent reason.