One of my favorite little ways to annoy people trying to collar choke me from the back: I drop my chin to my chest, sliding my chin along my chest as I look into my shoulder, scooping my collar up with my chin as I do so. It's even easier if I have a thumb and index finger free for a split second to pick up my collar. Sure, you get some scuff marks on the chin, and it can hurt a bit, but you can't really be collar choked with your gi over your chin.