Arctic Silver 5 or Arctic Ceramique are two of the best thermal pastes out there. If you have an Arctic Cooling heatsink, AC MX-1 thermal paste usually performs a bit better than Arctic Silver. These pastes can be bought from Newegg or many other stores. To apply, put a small dot (rice grain size) on your heatsink and rub with a piece of plastic wrap until there is a circle of different color in the bottom of your HSF. Then put a similarly sized dab on your CPU IHS and mount the HSF. Try to wiggle the HSF a bit to spread around the paste once the HSF is clamped in.