The method linked to works perfect, except I'd also add two things:
1) Make sure Scan Hang Time is set to 0. You want the radio to snap back to the conventional channel as soon as the talkgroup has muted. This doesn't give the radio a chance to do anything on the talkgroup, much less monitor the control channel.
2) Set your Revert channel to Selected Channel. This way, if you hit the PTT button, instead of trying to select a talkgroup that might be in use, it'll always snap back to the conventional channel.
The advantage of this method (scanning from a conventional channel) is that you can program the conventional channel to be the system's fall-back frequency, if it has one, so you won't miss any action if the system goes down.