First, the decoy spread should contain an opening for incoming birds to land in and that opening should be well within your shooting range. Next, the decoys and the opening should be properly placed in relationship to your blind. Ducks circle with the wind and land into it.

Since the opening is your killing zone, it should be placed on the downwind side of the blind. A useful variation is to place the opening in the decoy spread so that the wind is blowing across the front of the blind. This can help keep the birds from circling directly overhead.

I like to pull all or most of my decoys and rearrange them frequently. If possible I hunt alternate blinds. When ducks get shot at regularly from the same location, they get really wary.