My way is similar to bikerbabe883, but I have a link to some step by step photos.

Take a wad of wet paper towels and add a drop of dish soap. Heat this up in the microwave for 1 minute. Let is sit for 5 minutes and then use the paper towels to wipe down the microwave. The crusty food just falls off!

For step by step pictures showing this technique, check out this article: How to Use Wet Paper Towels to Steam Clean a Microwave

This technique is great for weekly maintenance and for stubborn food splatter.