If you use top notch hooks you (a) never need to sharpen to start with and (b) rarely need to sharpen in the field. I'd guess that over the course of a hard core season I needed to do it less than ten times(?)
When needed, I use a simple two-sided technique which follows the tip of the point with a very shallow skew angle. There is only a certain taper and diameter you can attempt, and once the point becomes too blunt you can't get it to sharpen effectively.
I like to break off the hook at the bend once it's beyond hope - these flies are great prospecting flies when you are looking for fish for someone you're fishing with. It's still fun to get the big grab and even more fun to say "cast over there I think you'll find a fish" >ThWoNk!<