TSP and scrub brush
After Soaking up the majority with rags and kitty litter, get some Trisodium phosphate (TSP) that you get for prepping surfaces such as interior walls of homes that need painting. Get it at your local paint dept for 3$ for a pint or so.
Pour a bunch on the stain add a little water to make kinda like a paste.
Scrub the paste well onto the oil spot and let sit for 20-30 mins.
Hose it off afterwards. Depending on the amouint of time the oil had to soak into the asphalt, you may need to repeat the process.
Note to all (no comments about my stupidity/neglect on what follows):
Last winter I had a larger spill

.
I had to do the above cleaning procedure to my asphalt driveway after having replaced oil in my beast but because it was like -100deg F outside and I didn't check to ensure the mating surface of the oil filter housing was ready to accept the filter.
It turned out that the gasket from the old filter had dislodged from the old filter and remained stuck to the housing and of course I had screwed the new filter ontop on the old gasket. As soon as I started the engine to check for leaks,

I had a frickin huge one within about 5 seconds as the oil spewed from underneath the filter onto the driveway, until I shut the engine off. Pretty expensive lesson to learn, having to buy more synthetic fluid and and all.
Note to self and all: check the mating surface of the oil filter every time replacing filter, not only if the oil drain plug has been installed prior to start up.