First, wipe the post and the inside of the post tube thoroughly with a cloth and a bit of rubbing alcohol. Then put a bit of oil on the clasp, being careful not to get any on the post. You may need to do this every few weeks - just make it part of your regular maintenance routine. Tighten the clasp hand tight with the screw before closing. This does the job for me, though I still sometimes have to readjust the saddle height after riding for about an hour, especially if the road has been rough, like the cobbled sections of your Hudson River path. It helps if I raise my rear off the saddle standing on the pedals for the biggest bumps - on a horse its called "posting".