One thing I've started doing- If I'm planning to significantly slow down at a non-obvious stopping point (either because I want to relax at a nearby park, or if I don't particularly want to hammer through a blind corner on a narrow MUP unlike the guy behind me on a TT bike, I like to very deliberately unclip one pedal and dangle the leg- I figure people get the message.
If this is some kind of severe bad idea, let me know, but I'm guessing nobody wants to ride behind someone who's going to suddenly slow down without warning.