When it comes to changes, the drummer is my salvation. I listen for vocal cues, melodic cues, and most of all drum cues.
Counting is definitely the best way to keep track, but listening for the drummer to telegraph a change works every time as well.
+1 Often, the drummer is already counting (either consciously
or unconsciously). He should be able to give you a little lead-in
to the change. And, even if he gets it wrong, you two are still
synchronized. So you go around one extra time. It won't matter.