Originally Posted by rickyroy
thanks everyone once again for all the help. I've tested it and it's definitely moving where the post meets the frame, not where the seat meets the post. Also noticed it getting lower too, not just side-to-side. It's a tough one.
Something you could try is roughing up the post a little to create more friction. And cleaning it as well. In another thread about new bikes someone had a descending seat problem that was resolved with a wipe to lessen the amount of grease on the post. In your case I doubt that there's much original grease on it but there really might be something to be had by scoring some vertical lines in the post with a metal scribe - the extra friction and mild displacement of the steel may well be enough to keep it tight. Maybe even just a little roughing with some sandpaper would be enough?
Failing that a makeshift shim fashioned from a pop can might work though please be careful it's tight before riding anywhere.
Who said 'Zen and the Art of Motorcycle Maintenence' didn't actually have some useful tips?