If you have a dishwasher, put them in the washer and as soon as the cycle is finished remove the racks. Using a wire brush or steel wood scrub them quick. The dishwasher softens up the baked on grease and grime. I find my dishwasher alone does not clean them.
I use purchased spray-on oven cleaner. Place one rack on the opened oven door and spray, wait a few minutes and wipe off with a wet sponge. Also wipe off the door. Then repeat on the other side. Now place the rack in bathtub or large sink holding hot sudsy water to remove any residue of the oven cleaner. Do the same process with the 2nd rack. This is so easy, no scrubbing at all and you're cleaning the oven door at the same time ! Make sure you have covered the floor nearby with newspaper .