K.P.
Good old-fashioned elbow grease! :-)
What I usually do is clean it as well as I can with water or dish-soap and water, and then all that's left is the stuck-on stuff. Then I sprinkle baking soda all over it, and pour white vinegar on top of it -- it will bubble and fizz! Then get a soft scrubby and scrub up what I can. The baking soda is abrasive enough without being too abrasive; and then I repeat it until it's clean (or as clean as I can get it). It's more work than using chemicals in a can, but it doesn't fume and I'm not worried about any possible cancer-causing substances in the commercial oven-cleaner that they haven't discovered yet.