Lapping.....
There's a pretty good guide to lapping contained in the 'Important Stuff: Advanced Guide to Overclocking and Cooling' sticky in this overclocking forum. As a beginner, I just used it to guide my first go at lapping and I recommend it.
The basic theory is that you take your cpu (and possibly heatsink/cpu water cooling block) and sand it down to smooth off the surface. You start with a low 'grit' wet 'n' dry sand paper like 400, and incrementally work up to something like 1500 or potentially higher. Obviously the smoother the surface of the cpu and heatsink then the better the contact will be between them. The better the contact is, then the better the heat can be transfered away from cpu to the heatsink, and consequently the cpu temp. should be lower.
If you decide to give lapping a go, i'd highly recommend going to Halfords for your sand paper as they do grits 400 through to 1500 (intended for car paint spraying). I found it a right pain to get them anyhere else.
