• Fill a coffee cup half full of water and add a pinch of salt. DON’T SKIP THE SALT!
• Microwave on high for 2 to 3 minutes
• Remove CAREFULLY!
• Wipe down the microwave with a damp cloth.
• Most of the gunk will easily wipe away and you can always repeat for the tough stuff.

 I don’t remember where I read this but it is hands down the best chemical free method I’ve found.

Oh, BTW the salt doesn’t actually help with the cleaning.  It’s to prevent the boiling hot water from blowing up in your face when you move it (very low on the probability scale but still - DON’T SKIP THE SALT!)