When your dog willfully disobeys, stare into his eyes, stand erect, square your shoulders and loom over him. When he drops his ears in submission, make him obey the original command. If you need to make a point forcefully, do what the alpha dog does when a subordinate does something that really ticks him off. Grab him by the scruff of the neck and give him a good shake, enough to raise his front feet off the ground. Save this for serious infractions.
Most trainers use whistles because they are easier for dogs to hear at a distance.
Also, whistles don't get hoarse, and they don't say things that offend genteel sensibilities. The key to successful whistle use is settling on a distinctive signal for each command. Establishing these signals was easy. I just laid them on top of existing skills. She was sitting on the whistle command alone within five minutes.