Some of my students have recently asked me how I got so uncanny good at chokes. They make me want to laugh because my skill really is very modest. But seriously, my choking skills really have come together lately, and I've started getting these crazy, eye-crossing chokes in randori a lot lately.
So, I thought I'd give out one tip that I think is mine to give. I say I think, because I don't ever remember being taught this lesson, though I don't delude myself into thinking I'v invented something here.
Do not attempt to grab the collar in any old place and by main force wrench it around uke's neck.
Place your hand directly on the artery, then gather the collar into your hand.
This way, you will get much better positioning of the choking hand, and the collar will serve to anchor you hand to the artery.
Perhaps another way of thinking about this is - all collar chokes begin as katatejime - set the one hand properly, anchor it in place, then add the second hand in when you get the chance.