I came across this video on another board and thought to add it to this discussion.

A Break Free Knot for Neck Carry
http://www.youtube.com/watch?v=864FSx_cTYQ

I didn't have much success with the overhand knot the author uses. Instead I used a nail knot and that seems to have enough tension for my tastes. A nail knot is similar to a hangman's noose.

Here's a link to the nail knot.
http://www.animatedknots.com/nailknot/index.php?Categ=fishing&LogoImage=LogoGrog.jpg