Here is the method I fave used since my start in this hobby (its similar to sweetand salts). I tie a nail not using 20# backing using 7-8 loops, trim the ends closely, then give a quick pass over with a lighter rolling the knot between my finger tips, (this sort of fuses the loose ends together).
I have yet to have one come loose on me.
It looks messy in the up close photos, but is barley indistinguishable in reel life. Although what I'm doing might not be the recommended best way (is also a quick and simple solution), This will be fourth year fly fishing loops using this method, and have been happy with my results.