First of all use a good non stick pan, second make sure egg mixture is beaten well, third to make a fluffy omlette add a bit of water to the eggs but not too much that it gets watery, 4th be sure to preheat your pan but then cook the omlette on a medium low heat, 5th cook your omlette with a cover to cook evenly, 6th saute any hard veggies first so you dont end up with undercooked veggies or over cooked eggs, finally dont over do your cheese remember when you fold over your omlette there will be 2x the amount
Sounds to me like the original poster is not using a lid to cover the omelette, as suggested above. The top of the omelette will stay soft & soupy, whilst the underside burns. I like to use a small skillet(8") and use a lid that is a bit smaller than the skillet. That way the lid sits closer to the egg which keeps the trapped heat closer to the omelette surface, and also keeps the lid from making an airtight fit....that way moisture can escape which will make for a firmer omelette. A glass se--thru lid will let you see what is happening. Resist the urge to lift a metal lid every 30 seconds to take a look - this defeats the purpose of the lid!
Another tip I use is putting whatever ingredients you are going to use in your omelette in the pan & saute them UNCOVERED first with a bit of oil, butter, etc BEFORE adding the egg mixture. Cook the mix just a few minutes in order to soften them up, then pour the beaten egg mixture on top, and cover it. This will also 'cook' out any unwanted excess moisture that would otherwise cause a runny omelette.