Ideally, like with high performance computer tuning, both heat conducting surfaces should be lapped and the thinnest possible layer of heat sink compound used. Lapping means using sandpaper on top of a flat sheet of glass to sand on, going progressively finer until you get to 2000 grit or so. You can then polish the surface. This is done to both mating surfaces. In order to put the thinnest possible layer of heat sink compound like arctic silver I put a tiny drop of the stuff, the get some lacquer thinner and use that and your finger to spread it to an even and very thin coat, let dry completely and press the two parts together. Using this method I've measured up to a 20F drop in temperature on my computer hardware. This is very significant temperature differences with no other change in cooling equipment, just lapping and spreading thin heat sink compound. It seems crazy to take a $500 processor and start sanding on it before even installing it but it makes a big difference.