A good general cleaning solution for the microwave is a mixture of 4 tablespoons of baking soda to 1 quart of warm water. This can be used to wash down the inside of the microwave oven.
Baking Soda to Control Odor
Just as you store baking soda in the refrigerator to control odor, you can store a box of baking soda in the microwave in between uses.
Baking Soda and Baked on Food
Mix a few tablespoons of baking soda with water in a microwave safe cup. Boil in the microwave for a few minutes. The insides of the microwave will be damp and easy to clean with a paper towel or dishcloth. This has the added benefit of removing odors that may be clinging to your microwave oven.