Pulling too quickly can cause a knot to seat improperly, and can create unnecessary friction that weakens the knot. Be careful to trim excess material as close as possible without damaging the knot or line. This not only provides a neater appearance but also allows the knot to perform better for casting, and to pass more smoothly through the rod guides. Be sure to inspect your leaders for abrasion, wind knots, or other damage that may cause the fish of a lifetime to break free. Be sure to remove any tippet material that may be curled or damaged from a previous fly before tying on another.
Most knots slip before they fail.
Be sure the knot is fully seated and you will reduce your knot failures. If your line breaks on a fish or snag, examine it. If there are small curls at the end of the line, the knot failed. Aside from damaged line or weak knots at other points, the tippet is typically the weakest section of any leader. The ability to tie consistent, quality knots where the fly is attached to the tippet and the tippet is attached to the leader is crucial.