Mending fly line
Effecient mending is an often overlooked skill for any fly fisher in moving water. You may be a great fly caster...you may be able to put the dry fly wherever you want it to go. Good..but, if you cannot get that fly to float naturally into the fishes feeding lane, you are destined to limited success. Mending is a primary skill for a trout fisherman regardless of whether you are fishing with a dry fly or a nymph/wet fly. This is because feeding trout will almost always prefer to hold in a limited area when feeding. This is especially true when there are more bugs in/on the water. Most fish, especially trout are very aware of their environment and typically are most "comfortable" in places where they can find several things: food, safety and cover and, whenever possible, a current that does not require too much effort or it (the current) is "feeding" bugs down to them. Aside from helping to achieve a drag free drift, mending also keeps your fly on the water longer, thus increasing the likelihood of hooking a fish.
How to Perfectly Mend Your Fly Line by Kirk Deeter (Field & Stream)
Video Pro Tips: How to Mend Fly Line by Tim Linehan Orvis)
A Mending Primer by Philip Monahan by (Midcurrent and Orvis)
8 Common Fly Line Mending Mistakes by Lewis Cahill (GinkandGasoline.com)
Mending Fly Casting Video - by Redington Fly Fishing