Here's my method. YMMV.
1) Use sweeping compound or kitty litter to absorb as much of the wet oil as possible. Multiple applications may be necessary.
2) Lightly wet the stain with water. Cover the entire stain with Tide or other super-kick-ass detergent. Lightly wet the Tide on top of the water. Let sit in the sun for ... well, until you feel like proceeding.
3) Wash the detergent off. Scrub the stain with a steel wire bush. Let dry.
4) Cover the entire stain with undiluted Simple Green. Let stand for an hour or so.
5) Rinse off the Simple Green. Rinse thoroughly!
6) While the stain is still a bit wet, scrub it again with bleach.
If you still have a noticeable stain you can't live with, use oven cleaner.
I have tried all kinds of commercial products to try to clean garage and driveway oil stains, but I don't think that, especially for the money, they are very effective. I've never tried a pressure washer, but with the right detergent, these might produce good results in one or two passes.