google earth.
cruise around on my bike seeing what i can get into.
hit bike trails (even the paved cycling trails) usually have good spots off to the side out of the way.
hit up BMX'ers and mountain bikers. find out where the out of the way off the beaten path jumps and trails are.
iv found a few good spots by going through holes in fences and getting into places i really shouldn't. that's how i found this place

no joke, riding my bike on the riverbed trail, looked up saw a hole in a chin link fence, climbed about a 10foot tall rock wall, went through the hole in the fence. boo ya bashers paradise!