1. Make a paste of baking soda and water, and spread liberally on the area to be cleaned.
2. Let sit for several hours or overnight for really tough grime. Keep moist by misting with water every once in awhile.
3. Wipe out well, and then spray the walls down with vinegar and let dry.
The baking soda requires even less scrubbing than conventional oven cleaners (you have to try it to believe it!) and the vinegar helps repel future spills.