Here is the information from The Green Life: "Keeping your microwave clean by covering food (a plate or glass lid will do) can make it last significantly longer. The vapor released when the food cooks permeates the machine's interior and can, over time, cause problems. Vapor also transfers grease and residue, resulting in rust and corrosion. As for bags of popcorn? Put them on an inverted bowl or plate. They contain foil strips that can warp the turntable."
Not sure how to clean your microwave? Place a bowl of water, with slices of lemon and a cloth in it, in the microwave and turn it on for 3 minutes. After this, the steam will have loosened any bits of dried up food, and you will have a nice fresh cloth, to wipe it down with. Magic!
I don't know about you, but I had NO idea that an unclean microwave could cause so many problems. Happy cleaning!