I've already seen this question: Problems with a slipping seatpost
The main difference is that I have an aluminium frameset and a forged alloy seatpost - lightweight clamp.
After every couple hundred miles on my commuter (particularly over rougher surfaces), I've found that the seatpost slides into the frame by about half an inch or so. After the second time I've noticed and readjusted it, I figured it might just be because I have a lightweight clamp. I over-tightened it past the recommended torque, but alas, it happened again. I then tightened it as far as I dared without snapping the bolt, but have now found that it has slipped another half an inch over the last 200 miles or so.
Is there some kind of paste / compound that I could use that, similar to the carbon paste, would prevent this slippage? I asked the manufacturer and my LBS about the carbon paste, but they explained that it may not be a great help, because the carbon paste is "designed to create a small amount of swelling of the carbon" on the surface of the components, thereby improving grip. Well, this is metal on metal, so it's different ... right?
Update: It is not oily / dirty / dusty and it is not old and worn
For completeness, I have this bike: http://www.planetx.co.uk/i/q/CBPXRT58ALRIV22/planet-x-rt-58-alloy-sram-rival-11-road-bike
Any advice other than drilling a hole through it an putting a bold in (I won't really do this) ?
Note: I tried to add a tag to the question for the correct English spelling for ALUMINIUM, but apparently I need more reputation for that. Right ... ;-)