Saw this on the GoodHousekeeping site:
Put a chopped up lemon, lime or orange and several tablespoons of vinegar and a cup of water into a large (about 4 cups) sized microwaveable bowl. Put this in your microwave on high until the water boils and the window is steamy - then let it sit for 15 minutes before you open the door and wipe out the inside with a cloth or sponge (I usually use paper towels for cleaning up honestly). Then you can just use a mixture of vinegar and water to clean the window if it's greasy - I use this mixture for regular cleaning a lot already. I bet your microwave will sure smell nice after boiling that citrus in there, though!
Might give this one a try!