Magnetic Mic is the ultimate microphone holder.  It consists of an adapter that attaches to your radio microphone and a high strength magnet base that you mount where you want you microphone to stay.

To use, just get your microphone close to the magnetic base and let go.  The mic will "leap" to the base and stay put until you pick it up.  

We swear by them.  Yes, they are not cheap, but they are handy.  We are working to reduce the price but I don't see them getting much lower in cost anytime soon.