Hadn't run across this in the couple hundred posts I'd read yet, so wasn't sure if it was known. I picked up this trick from my stunt kite flying days where we Prusik our control line attachment points to the kite's bridle. If you take a Prusik knot and roll it over, it forms a more grippy knot (at least, that's us kite fliers' opinon) that won't slide. You can easily roll/unroll to lock/slide the knot. Pics follow:

(Click for larger)
Unlocked for moving


Locked


~Dan