Until the engagement is for something other than "get out of my way/off the road" I can't be bothered anymore. Too much breath wasted over the years, too much aggro. Occasionally I'll bite but not so much anymore. I tend to simply ignore them/wave them past in the other lane.

Just ride like you are invisible. This way you are being 'aggressively defensive' (I like that phrase, thanks AZTR). When I was a messenger all if ever heard when coming close to been smeared...'I'm sorry I didn't see you', then the epiphany happened... ok they claim they can't see me so I should ride like they can't. This allowed me to be all over the same direction lanes at will simply by virtue of the freedom given to me by the drivers' themselves with their claims of blindness. I rode where cars didn't want to be, and made sure I was gone when the light turned green. All the while being as careful as I could to not obstruct (because he can't me, he might drive over my backside). As I got stronger and faster I became less and less afraid of cars, now I figure one has to fall straight the sky to get me. This does not mean I get complacent. It's not completely unlike white water rafting, big and little obstacles, flow/blockages causing flow elsewhere.

Ride like you belong there and you will, ask permission and you'll be crunched.