at least once a year. for smaller clean-ups I put a bowl of ammonia in the oven at night and leave it sit at least over-night. The next morning (or afternoon if I'm lazy) just remove the bowl and wipe out the oven. The fumes actually loosen up all the gunk and you can just wipe it off. For really tough spots on the oven glass, use a paste of baking soda and water, wipe it on, leave it sit for a bit, take a slightly damp sponge, and wipe it off, add a bit of salt for really baked on spots, to get rid of the last of the baking soda, spray white vinegar on it, watch it foam and bubble (remember that elementary school volcano), then wipe off with a clean towel. Polish the outside of the glass and any stainless with hydrogen peroxide, spray it on and wipe off with a dry cotton towel.