Go the the options menu, then cheats, and enter one of the following codes to activate the cheat next to it.
Warning: Some cheats could disable your Xbox Live Leaderboard listing.

FOREVERNAILED: 100% Branch Completion
NeedsHelp: 50 extra skill points
TheIndoorPark: Air And Space Museum level
GiveMeStuff: All create-a-skater items
LetsGoSkate: All decks
OverTheTop: All fun items
WatchThis: All game movies
SweetStuff: All Lounge bling items
LAIDBACKLOUNGE: All Lounge themes
IMGONNABUILD: All rigger pieces
LOTSOFTRICKS: All specials available
Trippy: All video editor effects
PutEmOnTop: All video editor overlays
ThePrezPark: FDR level
BeefedUp: Full stats
TheMissing: Invisible man
THELOCALPARK: Lansdowne level
TinyTater: Mini skater
AINTFALLIN: No Bails
MagicMan: No Board
STILLAINTFALLIN: Perfect Manual
AintFallin: Perfect Rail
CRAZYBONEMAN: Play as Boneman
MoreMilk: Play as Bosco
NotACamera: Play as Cam
TheCoop: Play as Cooper
Sketchy: Play as Eddie X
Piledriver: Play as El Patinador
Flyaway: Play as Eric
Rabbies: Play as Mad Dog
INTERGALACTIC: Play as MCA
NotADude: Play as Mel
LooksSmelly: Play as Rube
Movers: Play as Shayne
Dapper: Play as Spence
Shaker: Play as TV Producer
Booyah: Super Check
Myopic: Unlimited Focus
SUPERSLASHIN: Unlimited Slash Grind