I had this issue for a long time - 0.5mm to 1mm of slippage every km! Carbon post, alu frame. Clyde, 300 lbs.
Tightening it did not do much. At one point, I noticed the edge of the slot in the frame had worn a groove in my seatpost! I sanded it smooth (only the upper decorative layer was damaged - I caught it in time), added a soda can shim (due to the sanding making the post smaller) and bought a new clamp (bolt, instead of my old quickrelease), which I tightened down REAL right. As in 3-4 turns beyond "dangeous-seeming tight".
It still slipped!
However, the soda can shim sort of crumpled due to the slippage and this caused enough of an obstruction to stop it from slipping after a cm. I find the height pretty comfortable, so I am not going to mess with it anymore until it becomes a problem again
