What I had already done was thoroughly clean the seatpost and the inside of the post hole with alcohol, not soap and water. I also lubricated the Q/R mechanism. This does help considerably, because I only have to readjust the seat about once an hour, rather than every 15-20 minutes as I did before I did this. I also was tightening the Q/R too much, and it DOES damage the Q/R - stripped the screw - I had to buy a new lever and screw.
Haven't tried the suggestions here yet, but I will - probably try lubricating the shim this weekend.