Slipping aluminium seatpost in aluminium frame 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 

With this seatpost: 
http://www.planetx.co.uk/i/q/SPPXSLT3D/planet-x-superlight-team-3d-forged-alloyseatpost

And this clamp: 
http://www.planetx.co.uk/i/q/SPPXSLT3D/planet-x-superlight-team-3d-forged-alloyseatpost

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 ... ;-)
I had an exactly same problem: Ritchey WCS seatpost, Merida frame (both alu alloy) and a lightweight clamp, like in @Frisbee's answer. What I tried:


Beefy clamp with various torque settings. Didn't help at all.
PVC tape. Helped, but didn't last long.
More torque on a lightweight clamp. I snapped it in half with only 6Nm.
Hair spray. To my surprise, it worked and seatpost stopped slipping, creaking and annoying me.


I've switched to a CF seatpost since then, same problem occured, but hair spray come to rescue again. No slipping for months.