I don’t fancy buying any bike-specific products. I might as well just buy the correct-sized seatpost then (if that makes sense).

lose weight

loosen clamp, lower saddle as much as possible, tighen clamp. No more slippage.

or Aluminium Foil, a couple of turns round the seatpost.

DEvils work- shims.

carbon paste or mix some chalk dust in with some grease.

It’s worth buying some Carbon Paste. A wee tub will last ages, even for serial bike swappers.

Surly Constrictor.

Piece of a coke can

a Jubilee clip. tacky but worked for me – or a seat clamp with an alen bolt seemed to be able to get that tighter than a quick release.