A lot of emphasis is usually put on efficient mowing, but once you have squeezed all the time out of that, you will find that the trimming operation can be surprisingly refined.
Use top quality line. (cuts better, lasts longer).
Get rid of the guard.
Learn to keep moving: a sweeping rythem as you continue to walk along.
If there is only a small amount of edging, I often "cheat" by using the line trimmer, rather than walking back to the trailer for the stick edger.
I usually divide the property into quadrants: right side, back, left side, front. I move clockwise around each of these areas, starting along the house, then moving out around the perimeter, then catch whats in between, ending up at the beginning of my next quadrant. I end up in the
front, near my truck (if I can).
Don"t look back, it will never be perfect anyway (just glance at a general area when its done).
My attitude is that I am more critical than my customer; if it doesn't look like it needs trimming from a few feet away, it doesn't need trimming.
By being less than a perfectionist, I have cut my trimming time in half from the early days.
Finally, I am a proponent of mow, then trim (I work alone). You don't know how little you may have to trim if you do i first. The 21" is never used for trimming. You can cover a lot of ground when you have to with the trimmer, especially when you let out the line.
Above is my approach, I'm sure everyone has their own best way.