It is important not only to select the right knot for a particular job but to tie it properly. Poorly Tied knots will mean lost fish and aggravation. Here are a few basic steps to follow when tying all knots for fly fishing.
Lubricate knots: Before you tighten a knot, lubricate it with saliva or by dipping it in the water. This will help the knot slide and seat properly. Lubrication also decreases excessive heat which dramatically weakens monofilament. Heat is generated by the friction created when knots are drawn up tight.
Seat the knot: Tighten knots with a steady, continuous pull. Make sure the knot is tight and secure. After it is tied, pull on the line and leader to make sure it holds. It is better to test it now than when a fish is on.
Trim neatly: Use nippers to trim the material as close as possible without nicking or damaging the knot.
Also, some knots such as the Duncan Loop or Uni-Knot, after repeated use, weaken the line after repeated use due to friction and can weaken the line up to 50%. Be sure to check your line and re-tie when neccessary.