To use ssh multiplexing for host example.com:

Add the following section to {{{~/.ssh/config}}}:

{{{
Host example.com
    ControlPath ~/.ssh/control.example.com
}}}

The ssh client will now use the socket at the specified !ControlPath to
create new connections to example.com.  A connection will fail if it is
attempted and no master connection provides this socket.

Before attempting a connection, use the following script:

{{{
if test ! -e ~/.ssh/control.example.com ; then
  ssh -fMN example.com
fi
}}}

If the control socket does not exist, this will spawn a master (-M)
that executes no command (-N) and goes to the background (-f). If the master is killed with anything nicer than {{{SIGKILL}}}, it will remove the socket. If the master is killed brutally, it is necessary to remove the socket manually.