I actually researched this over this past summer. Some friends and I were going to Shakespeare in the park and I wanted to make bags of popcorn for us all to enjoy during the show.
There are 3 keys that I found.
One is use the right kind of salt. You can buy actual popcorn salt, of if you're like me, just make your own. The key is that the salt must be powdered. So you can buy it, or put your salt through a grinder until it has a very fine, powdery texture. (Be careful not to inhale it!)
The next key is to use coconut oil to pop the corn. If you want really evenly salted popcorn, add your powdered salt to the oil as the popcorn heats. You can use a hand cranking popper, or a regular pan with a lid - just be sure to keep the corn moving so the heat is evenly distributed.
The final key after your corn is popped? Clarified butter. If you just use regular butter (or margarine) it will make the popcorn soggy, but clarified butter will NOT cause sogginess. You can buy clarified butter, or make your own. To clarify unsalted butter, melt it slowly and thoroughly, without bringing it to a boil. Let it sit just a bit, and the water, solids and fat will start to separate. The water will evaporate, and most of the solids will sink to the bottom. Any solids that float can be skimmed off, and the liquid left behind is the butter fat. That's what you use for the popcorn. Butter flavor without the sogginess!
All that being said, you can buy fake butter flavorings, like Flavacol, which a lot of theaters probably use these days. But, if you want the pure stuff, it's clarified butter all the way!