Become a "duck psychologist"
Humans have a body language, and psychologists learn to read fidgets and other subtle cues that reveal when patients are nervous, comfortable, bored, etc.
This same concept applies in duck calling. Hunters should learn to read ducks' body language and utilize this in their calling. They should watch for positive responses that indicate the ducks are anxious to join their decoys. Conversely, they should be alert to what scares ducks or causes them to be cautious about coming in. This ability to "read" ducks comes through experience aided by instruction from such experts as Steve Barnett, Richie McKnight, and Bryan Hanson.
As Steve Barnett advises, learn to be a good "duck watcher" to get inside the birds' minds and figure out what calling approach is working best. Truly, this is the main difference in being an effective caller and an ineffective noisemaker.