The emery cloth and shoe polish method is the way I go, also. The trick is to do a wee bit and move, do a wee bit and move, ensuring that you do your best to do the entire post as close as the same amount as you can. During the process, use your calipers to check diameter in several places. It is important to avoid flat spots or out of round. Area of contact is what holds the post in place, not just clamp pressure.

So, once you have cleaned and polished the post to a uniform thickness, you should be good to go. However...

As others have mentioned, if you have to remove too much, then the post will slip and you will, most likely, distort the seat lug by over tightening the seat post clamp bolt.

Anyway, this has worked for me many times, over the years.