A great way to clean your microwave is to boil a solution of 1 Tablespoon of vinegar, one cup of warm water and 1 or 2 drops of dish soap in a small microwave safe bowl.
Put it in the microwave on high for 3-4 minutes and let stand for 15 minutes. Then wipe the microwave out with a damp sponge. The steam and use of vinegar loosens up the build up in the microwave. The solution loosens splattered on food and deodorizes.
Warning: It is important not to heat fluids in your microwave for long periods as they can become super heated, this can cause an irruption within the fluid even after the vessel has been removed from the microwave. This irruption can send scolding super heated fluid in your direction.