From our good mate and custom fly designer Davy Wotton, come the Davy Knot. It's a 100% knot that ties very easily once you get it down.
Davy also offered some tips on tying his knot on John Wilson’s Fly Fish Arkansas and Missouri Board
There is one major point to remember when you snug the knot taught. Always hold the tag end and draw the knot down by pulling the tag, while holding the upper. As a rule l hold the fly and tag at the same time.
In the case of using a larger hook and fine diameter line, when you make the second over and under turn, do the same again, the knot will not slip, l have used this for tarpon and many other saltwater species.