Easy-Off Oven Cleaner. Then hose it off. Works great, but be careful where the runoff goes. It will nearly kill any plants it touches, but in a few weeks will be a super fertilizer for those same plants. The kitty litter is the same thing as "oil-dry" but a little cheaper. I'd use the kitty litter (the cheap stuff only) first, then hit it with the oven cleaner if you're not satisfied.
