I did not read this entire thread, but I will. I wanted to relate this to golf...and my brother, who taught me to swing a club. He told me that I have a distinct advantage, being 6'4" tall. The longer the arc of the club head, the farther the ball will fly, however, the more room for error in relation to where the club head hits the ball. Well, with throwline, that's much less of a concern. I took this analogy, and incorporated it with a technique that my friend Tim C. took the time to teach me.
Tim throws with his R hand only. He ties a stopper knot well away from the throwbag (like 4'-6' away...). The trick is keeping tension on the bag diurnal the wind ups. Your hand sort of hits three points of a triangle; when the bag is in front of you, your hand is about as far out in front as well (point 1). As the bag begins to swing back, you need to raise your hand to keep the bag off the ground (point 2). When the bag swings out behind you, you need to allow your hand to follow it (point 3).
I recommend to practice the above technique, without concern for releasing the bag...just learn to create a long, slow arc while keeping tension on the bag. When you're comfy with that, think about release, but do it in the wide open (no trees around). Don't put any body into it...let the length of the arc do all the work for you and you will be amazed at how little effort is required to attain incredible height.
The part that is more difficult to explain is the release time. One thing TM also taught me was that a 45 degree angle to your target is ideal. H related it to the military and mortars. If you have the room, do it at 45...if you don't, just get as close to it as you can.
There is never any perfect scenario, but just remember and practice this: the longer the arc of swing, the farther/higher you can throw AND, the more time you have to determine your release point.
I sure hope this helps, and that I haven't repeated too much of what has already been said in this thread. Everyone has offered great advice (at least on page 1)