Before you make a bad situation worse by using a beer can shim try this:
1. Thoroughly clean the inside of the seat tube and the outside of the seatpost with rubbing alcohol -- you want both surfaces CLEAN and dry. If you can slip off the clamp collar, do so and clean it 00 lightly grease the threads of the bolt and nut/treaded area of clamp.
2. Spray three to five LIGHT coats of cheap hair spray on the part of the seatpost that will be inside the seat tube, makeing sure you get the clamp area. Let the hair spray dry well between coats.
3. Carefully assemble, trying to pus the seat post straight down, not rotating it from side to side. Torque the bolt, being careful not to overtighten and crush the cf seatpost.
This works wonders on handlebars that want to rotate, esp. cf bars and stems that you don't want to over tighten.
Terry D.