Bill, a very interesting topic, First off, try not to ever let anyone else use your mic, if by any chance you do, always carry a
little can of Lysol in your bag and give the mic a quick shot...I do this before every performance just to kill "mold" growth that can happen overnight on your mic from the moisture of your breath, especially if you use the rubber filter screens on your mic...
the other thing to consider is are you holding the mic while singing or are you using a mic stand or boom while playing the keyboard...this will make a difference in how close you are to the mic.....remember, the mic is an instrument just like the rest of the instruments on stage, you have to learn to control your voice volume with the mic as to farther or closer to your mouth for emphasis and phrasing effects...if you leave the volume level on your amp or keyboard the same at all times, you will soon learn to use your mic to control the volume with out "EATING" the mic, which while doing so by the way, tends to make everything you say or sing distorted and garbled anyway...that is for people that can't sing IMHO...I try never to hold my mic less than 6 in from my mouth unless singing very soft or low for effect...I hope this is of some help to you.....
TR