How do I get seasoning to stick to home-popped popcorn?

If you want dry seasonings to stick to popcorn, you will probably need to add a liquid to adhere them with. You could try adding butter or oil to your popcorn while it is hot, then adding the salt and tossing it together. If you're avoiding extra fat, a few spritzes of a non-stick spray (like Pam) might do the trick without adding significant fat.

I air pop my popcorn and then I spray it with Dr. Bragg Liquid Aminos. It has a salty flavor and if you spray it lightly it doesn't make the popcorn soggy.

If all you want is the salty flavor then this will do it but it also has the added advantage of allowing other spices to stick as well.

In addition, as Sourd'oh says, to using some fat as glue. Water will also work if you don't want calories, but will soften the popcorn somewhat.

Use popcorn salt. This is salt that is very, very fine, so that it sticks more easily to popcorn.

Additionally, applying the salt immediately after popping when the popcorn is very hot will help.

Put the salt in a spice grinder (such as this one)! My grinder has several options for granule size. Unfortunately, it has no markings or logos on it so I can't tell you what the exact type it is else I'd highly recommend it.

The problem is not liquid. The problem is the size of the salt. You have to grind the salt. If you don't have a spice grinder, (I use a mortal & pestle) you can put a cup of salt in a blender :) and use it in the future.

Why not add the salt to the oil while it is heating up - or otherwise mix with the kernels before throwing them into the oil? I have done this both with sugar and salt and it works well for me (and the salt does not appear to burn or damage the saucepan... If you shake everything up (as you should do) while cooking, then the popcorn gets evenly coated. However, no-one else seems to suggest this, so there may be something I am not considering...