The best solution is cool product called Pour-N-Restore. It goes on as a liquid and dries into a powder. The company's website claims that the product soaks into the concrete and pulls up embedded oil stains, capturing them in the dry powder. Whatever it does, it works! I've used it several times on my driveway and I can't even tell where the stains were.
Another cool thing about this product is that you're not rinsing oily slime off the driveway into the grass or down a drain. Pour-N-Restore holds the oil in the powder, so you just sweep it all up. The back of the label also states that it's biodegradable and phosphate-free.