To make the greatest omelet in the world, just make sure that the eggs are at room temperature by leaving them out of the refrigerator for 30 minutes before using them. Cold eggs are too stiff for an omelet.
If you always add a little milk to your omelet, try adding a small amount of water instead. The water will increase the volume at least 3x more than the milk. The water molecules surround the egg’s protein, forcing you to use more heat to cook the protein and make it coagulate.