The best maintenance is preventative.  Protecting your microphones from environmental pollutants is the best insurance against loss of performance.  The air is full of contaminants such as dust and cigarette smoke.  These can settle on the diaphragm and degrade its frequency response.  What's more, breath moisture and humidity can short out the high impedance parts of condensor microphones, causing thin bass or crackles.  Let's look at several ways to preserve your microphones.

Pop Filters
Although the main uses of a foam windscreen are to reduce proximity effect and give color to the sound operator, the filter also shields the microphone diaphragm.  Breath moisture and food particles have a hard time making it through the filter.  But remember, windscreens decay over time.  They slowly turn to powder which may collect on the diaphragm.  Periodically replace old filters with new ones.

Keep Them Covered
If you leave a microphone on its stand without a cover, dust settles on the microphone capsule and gradually degrades its response.  After each session, put the microphones in the pouches or cases.  For long term storage, put them in a closet or cabinet.

No Blowing!
Never blow on a microphone for a mic check.  Doing so can force particles through the grille screen and onto the diaphragm.  In many microphones, the diaphragm is made of very thin metal foil.  A breath blast can bottom out the diaphragm onto the backplate, causing a spark, which may perforate the diaphragm.  To test a microphone, simply talk into it or scratch the grille.