4. ESTIMATE RANGE CORRECTLY
Not all of those annoying hunters that jump up to shoot before the ducks are in range are doing so simply to beat you to the bird. Many are genuinely terrible at judging distance. Hopefully you aren't one of them. But, in your defense, judging range to ducks can be very tough. Both horizontal and vertical range must be determined, and oftentimes—especially over water—there are few visual reference points.
The key? Learning the relative sizes of the birds themselves, which don't change. This is more of an art than a science, and experience is the best education. I find that on a clear or slightly overcast day, when I can see detail on the birds, they are in range. That means identifying the colors of a wing speculum or recognizing individual primary feathers.
Another trick: Place a couple decoys 25 yards from your shooting position for reference.