I put this info in a post on overclocking a couple days ago. It comes from MaximumPC magazine and may be of help.

"Also mentioned is a technique for improving cooling. Most heatsinks lack a smooth finish on the base side of the cooler. They suggest using 800 grit moving to 1600 grit polishing paper attached to a glass surface. Use the polishing block in a figure 8 motion to evenly sand the heat sink. A mirror like finish will improve cooling efficiency."