Hold down L1, L2, R1, R2 and enter the following codes at the 'character select' screen for the desired effect.
All Boards:
X, triangle, O, square.
Level select:
Triangle, X, square, O.
Get $99.00:
Triangle, X, O, square.
All skaters:
O, triangle, X, square.
Fuzzy skaters:
X, triangle, square, O.
Big head Homer:
O, X, triangle, square.
Underwear Homer:
triangle, O, X, square.
Big head Bart:
X, square, O, triangle.
Gangsta Bart:
O, X, square, triangle.
Demon Marge:
X, square, triangle, O.
Big head Lisa:
square, triangle, X, O.
Gangsta Lisa:
square, triangle, O, X.
Big head Nelson:
triangle, square, O, X.
Ballerina Nelson:
triangle, square, X, O.
Sunglasses Otto:
square, X, O, triangle.
Big head Frink:
square, X, triangle, O.
Groovy Frink:
X, O, triangle, square.