Make a paste with vinegar and baking soda, and let sit overnight. Then scrub scrub! If there is stubborn grime, you can scrape with a razor blade.
If you have a lot of grease in your oven, try Spray Nine (Home Depot or online). Really spray it on heavily. Let it sit overnight and use paper towels to wipe away the dissolved grease the next day. Spray again and wipe with paper towels again. If this doesn't get everything, repeat the overnight soaking, or scrape the rest of the residue. Then use mild soapy cloth to wipe down the oven (particularly the heating element) then use a clean wet cloth for the final rinse. I use this method when cleaning really bad ovens in our rental properties. They come out spotless.
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