Reprinted fromhttp://repairfaq.ece.drexel.edu/sam/micfaq.htm#micrra

Most microwave ovens do not really need much in the way of maintenance. Regular cleaning is probably the best help you can give a microwave oven and will help avoid costly repairs in the future.

Clean the interior of the oven chamber after use with a damp cloth and some detergent if necessary. Clean the exterior of the cabinet and touch pad in a similar manner. DO NOT use a spray where any can find its way inside through the door latch or ventilation holes, or a dripping wet cloth. Be especially careful around the area of the touch pad since liquid can seep underneath resulting in unresponsive or stuck buttons or erratic operation. Do not use strong solvents (though a bit of isopropyl alcohol is fine if needed to remove sticky residue from unwanted labels, for example).

Inspect the cord and plug for physical damage and to make sure the plug is secure and tight in the outlet - particularly if the unit is installed inside a cabinet. Heat, especially from a combination microwave/convection oven or from other heat producing appliances can damage the plug and/or cord. If there is evidence of overheating at the outlet itself, the outlet (and possibly the plug as well) should be replaced.

Periodically check for built up dust and dirt around the ventilation holes or grills. Clean them up and use a vacuum cleaner to suck up loose dust. Keeping the ventilation free will minimize the chance of overheating. 

Listen for any unusual sounds coming from inside the oven. While these appliances are not exactly quiet, grinding, squealing, scraping, or other noises - especially if they were not there when the oven was new - may indicate the need for some more extensive maintenance like belt replacement or motor lubrication. Attending to these minor problems now may prevent major repairs in the future.

Keep your kitchen clean. Yes, I know, this isn't exactly microwave specific but cockroaches and other uninvited guests might just like to take up residence inside the electronics bay of the oven on the nice warm controller circuit board or its neighborhood and they aren't generally the tidiest folks in the world.

If it is too late and you have a recurring problem of cockroaches getting inside the electronics bay, tell them to get lost and then put window screen over the vents (or wherever they are entering). Such an open mesh should not affect the cooling of the electronic components significantly. However, the mesh will likely clog up more quickly than the original louvers so make sure it is cleaned regularly. If possible, clean up whatever is attracting the unwanted tenants (and anything they may have left behind including their eggs!!).

CAUTION: Do not spray anything into the holes where the door latch is inserted or anywhere around the touch pad as this can result in internal short circuits and costly damage - or anywhere else inside, for that matter. If you do this by accident, immediately unplug the oven and let it dry out for a day or two.