If you use oil for popping your popcorn, the seasoning will stick to it easily. However, to get flavor and stick to air-popped popcorn without fat, you can mist your popped popcorn with a bit of water or some other liquid.
Sprinkle your popcorn as you move it to the bowl with a little liquid spray bottle. This way, each layer of popcorn will get a little spritz, making it easier for the flavoring to stick to the popcorn.
Alternatively, drizzle honey or maple syrup over the popcorn as it falls into the bowl and then dust with your preferred seasoning.