Slipping aluminium seatpost in aluminium frame
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.