Key to your CPU's ability to stay cool is to use the right amount of thermal paste and apply it correctly. In other words you want as much thermal conductivity between CPU and the cooling block.
Use a quality thermal paste and apply it in a manner that prevents air pockets, spreads evenly, thinly and thoroughly between the CPU and cooling block.
There are two metal surfaces that will make thermal contact. Make sure both surfaces are clean. If your unsure, clean both surfaces with 99% alcohol and a dust free cloth. A coffee filter works great. The alcohol will naturally evaporate in a second or two. Then we'll "wet" one surface and "mud" the second.
When wetting a surface, your finger is best and easiest. Clean your finger with 99% isopropyl alcohol, and when dry, dab your finger with a small amount of thermal paste. Spread it as quickly as you can, but be sure you wet 100% of the surface. How thin is a properly wetted surface? If you can see the metal surface beneath, and also see evidence that it covers the surface entire, you did it right!
Mudding the second surface is easy. Dab a small amount of the grease and use a credit card and begin spreading the compound around. The edge of the card will pick up excess paste and can be reused to fill any area that doesn't have enough of the paste. Spread it as thinly and evenly as you can. Put it on to thick allows the paste to boil and form bubbles. When done, it'll be too thick to see the metal below and look evenly spread across the surface.
When you join the two surfaces 100% contact is almost a certainty. paste on both surfaces will naturally attract each other, and not allow air bubbles to form.