If you've been scrubbing oven racks with little success, let hot water do most of the work for you. Place an old towel in the bottom of your bathtub to protect the surface from scratches, then lay the oven racks on top of the towel.
Fill the tub with enough very hot, preferably boiling, water to completely cover the racks. Pour in about one cup of powdered or liquid laundry detergent and gently stir the water to dissolve it. Let the racks soak for at least four hours — overnight is even better.
Remove the racks and scrub them with a stiff brush or non-scratch scrubbing pad. Most of the residue should come off with very little effort. Rinse thoroughly and let the racks air dry before putting them back into the oven. One final reminder: don't forget to clean your bathtub afterward since all that grime has to go somewhere!