Ok, one thing that people avoided is the initial statement. We all know that unless you buy a used Heatsink, it comes "pre-lapped." But what if you eventually down the road want to maybe put in a higher grade thermal paste or you buy a used heatsink, or whatever the case may be. You need to perform what's called "Lapping" or "Re-lapping" of the heatsink.
This is usually done with 1500 grit sand paper, then clean it off, then 2000 grit sand paper, then clean it off. And lastly, 3M's rubbing compound with a microfiber or something of the sorts. Then clean it off again. Make sure to use 99% isopropyl alcohol which you can pick up at CVS or Walmart or wherever. As far as the sandpaper goes, Wal-mart, Autozone, or a car parts store should have them, along with the 3M Rubbing compound.
I've performed these steps many of times and keep a good stock of the items required for this around here.