There are a few different methods to making fluffy omelets. The first uses a little water or milk incorporated in the eggs.

You break the eggs into a bowl then wisk them till they are mixed completely, adding a tablespoon of water per egg, then pour them into a omelet or saute pan that is preheated with your fat of choice(butter, olive oil) Use a spatula to move uncooked portions of egg into the center. cook till eggs are half done, salt and pepper if desired, then cover with a lid for 40-45 seconds.Remove the lid, then take your spatula and carefully turn the eggs over, add fillings if desired, and fold the omelet to close it. Cover again for 30 seconds.

Another method for fluffy omelets is this.

When breaking the eggs, seperate the yolks from the whites.Blend the yolks together, take the reserved whites and whip them till they increase in volume, then fold them into the yolk mixture. Pour this mixture into your preheated and oiled pan. Cook them while using the spatula to keep moving the cooked bits to the edges, then when it is half done, cover with a lid. After 40 seconds add your fillings, reduce heat, then put the lid back on for 40-60 more seconds.

Another method requires a oven safe pan and a oven.

Preheat the oven to 400 degrees, Beat yolks, add 4 tablespoons milk and seasonings. Fold in stiffly beaten whites. Melt butter in heavy pan and add egg mixture. Cook omelet until it sets, then put in 400 degree oven until it is fluffy.