On the Multiplayer/Classic map The Mall, there's a ramp area past the rail where you spell out Combo (or, if you didn't do the Classic, it's below the 16 set of stairs).

Near the left of the ramp is a hard to see seam. Go up the ramp and pull RT while pushing up on LS.

You should slide into the wall.

Here, you can do any trick without worry of fail. I pulled a 6 800 000 combo once.

To get down, simply dismount (LB+RB).

If you wanna go for an ultra high score, while the combo timer is still ticking, pull a manual and go nuts with flatland tricks.