Pop your popcorn. I don't have an air popper so I put about 5 tablespoons of kernels in a paper bag.... seal the top after pressing the air out and pop on high for about 5-6 minutes. It doesn't pop all of the kernels, but it works well enough.
Once your popcorn is popped, sprinkle it alternately with water and salt. I do this in the paper bag while shaking it. You want random pieces of popcorn to get small wet spots. Now enjoy!
I know this sounds weird, but as I was trying to figure out a way to get the salt to stick without butter or substitutes I realized that with a little salt popcorn already has a really buttery flavor to it. The water brings out the flavor and a few little moist spots gives it the texture. Give it a try!