A crisp line mend is an important skill for a wet fly fisherman. Some of you may be well ahead of me, others maybe not. Here's the method I use:
To actually make the mend - Point the rod directly at the line, and focus on the rod tip. Draw a circle of about 1 foot diameter very crisply, ending up with the rod tip back in it's original position at the bottom of the circle. You can draw the circle starting at the left or the right depending on the river flow direction. With practice, this becomes second nature.
Making the line mend where you want it to - Sometimes you will want to mend the line near you, and sometimes farther away, depending on the current tongue you are trying to defeat. If you want to mend near you, have only a few feet between you and the point where the line first touches the water. If you want to mend further away, lift the rod tip until the line contacts the water further away. In general, the first 10-15 feet of the line in the water will mend. A haul pickup is a great help in doing this.
A haul pickup is a way to lift a lot of line off the water without making a commotion on the surface. Basically, you smoothly draw the line toward you with your line hand, while lifting the rod with your casting hand. If lifting the line for a backcast, lift the rod tip to about 10:00, then flick the line behind you as you normally would. If making a mend, use the same procedure, but don't raise the rod tip much above horizontal. Do not hesitate between the pickup and the mend.
I hope this kind of post is OK. Up to our behinds in snow here in central PA