The picture by slowhike illustrates how the larks head hitch can pull the rings unevenly. Granted the forces on the buckle by the webbing also tend to pull the rings uneven, but the larks head, in my experience aggravates the problem.
The webbing that slowhike is using seems to be very flexible and I find that helps greatly to reduce slippage.
slowhike - is that seat belt webbing? It looks like the color of most seat belts I am familiar with. 1" polyester seat belt webbing would really be ideal - very flexible and very strong with all of the advantages ofpolyester.