I found this cool new glitch that i hope you guys will enjoy. You need only your skateboard to do this.
1) Run over to the greaser's territory (auto shop) and head behind the building towards the garage.
2) Climb the ladder up to the roof, walk to the other side of the roof and turn around so you're facing the ladder.
3) Pull out your skateboard and tap X to gain speed. Before you slide down the slope of the roof, ollie and hopefully you will land on the roof opposite the ladder/where you just came from. That's the objective.
This in itself is a glitch because you're not supposed to get up here. A cool thing to check out is if you get to the second roof and try to jump onto the big roof just to the right (near the telephone pole) you will fall through the roof to see that there is no inside - you'll just fall into...well, nothing and get knocked out due to unknown reasons. This is similar to the 'Blue hell' glitch found in the GTA series.
I plan on finding more glitches soon, so keep an eye out and toy around with this one, see what else you can do from up there.