put some water and a lemon cut into quarters in a shallow bowl and heat on high till the water boils and steams. Stop the microwave but do not open it and allow the bowl to steam for a few minutes. Carefully remove the bowl and wipe over with a damp cloth et viola a clean sweet smelling microwave with no scrubbing
forgot to add that if your microwave is really crusty (don't ask...) you may need to do the steaming bit 2 or 3 times. you can use the same lemon and water. persevere it does work
Place a jam donut (not sure how this works, but it does) in the centre of the microwave for 10 minutes on high, the steam from the sugars mixes and softens the dirt on the inside edge of the microwave, making it dead easy just to wipe off.
do not nuke a jam donut for 10 mins on high in a microwave all the jam and sugar will get nuclear hot definitely weld itself to the surfaces, possible burn and weld itself to the surfaces, possibly burst into flames.