This is one of those pieces we should have told you about way back in the beginning, but didn't. There are one of two possible reasons for this.  We cefudk up (no swearing on this site)  We knew it all the time but thought it would be funny to keep it a secret. Obviously it's .
So what is "line management" and why is it something you'd want?
Allow me to take you on an imaginary journey... you've been carefully walking up the river - although it could be a lake of course, but since this is the Flow we'll call it a river. So far we haven't seen a snifter but suddenly and immediately, appears the largest trout on the planet, eating duns and since this is imaginary New Zealand you have exactly twenty three seconds before he buggers off or spooks for no apparent reason. In your haste to beat the clock you tangle the flyline around the tip ring, leave most of the leader in the gorse and hook yourself in the thumb. That would would be an example of poor line management.
First of all you have to be ready. For me this means hooking the fly to one of the rings halfway up the rod, with the leader passed around the reel seat. In an ideal world the end of the flyline would be outside the tip ring and this does in fact come recommended if at all possible, but since it is rarely possible - at least not with any leader length I choose to use - then you won't find me recommending it. Instead I'll recommend making your needle knot flyline-leader connection as small and as smooth as you can. Two and a half turns is all that's required for trout fishing (if your imagination has managed to stretch as far as saltfly then you'll be needing 5 turns according to Bruce Richards who is a bit of an expert, he says). A dollop of Aquasure or Loon Knotsense could be applied to the knot to make it smoother. I don't do this because it's a bitch to change leaders, but you can if you want.
I don't know if you degrease - many don't - but if it was I walking the imaginary river I would have been degreasing absent-mindedly from time to time. Your dryfly would have been greased up and ready to rumble right from the start of course.
As soon as you see your fish you should get into position, sometimes you already are in position, mostly you're not, but since I've already written about this in Plan of Attack you'll know which one applies to you. What you should do now is unhook your fly and flick it onto the water. Pull some line off the reel and use the tension of the water to pull this line out through the tip. You can also do this by sliding the rod tip through the water away from the flies (especially if there's not much flow around).
Pull more line off the reel - at least as much as you are going to need to make the cast - allowing some to slip downstream. I would be doing this while moving into position of course, trying not to fall in. With leaded nymphs you may have to roll cast downstream to stop the flies snagging and you'll have to be a little careful with what you are doing - but next up comes the Snake Roll. You don't have to make the Snake Roll, you could make a Tension Cast, but an aerial Snake Roll makes the change of angle easier and you can go straight into an overhead delivery and deliver the nail.
This is something to practice, practice, practice until it becomes second nature and I can't believe we haven't talked about it before. I blame Lars.