One of the most annoying things I've encountered over the years is twisting and turning seats. Years ago I started welding the seat clamps together where I wanted the seat to stay. Lately I've just been making heavy duty clamps with an extra hole to stabilize my seats.
This one is made from a piece of thick wall 1 1/4'' box tubing. A hole for the seat post, one for the pinch bolt and an extra one to keep it from swiveling.
Very stable. Most bike frame seat tubes are about 20 degrees off of level perpendicular.
Please tell us how you address this problem. as I'm always looking for better ideas.