Thermal compound. I use Artic Silver 3. Too much can be just as bad as not having any.
I just put my new mobo and AMD Barton processor together.
I used q-tip and denatured alcohol to clean the heat sink and cpu. Then use a lint free cloth to remove any small q-tip fibers.
Place a very small dab of thermal compound on the heat sink. Use a plastic sandwich bag to rub the compound center area that will contact the CPU. The layer left from rubbing should be only enough to fill in the microscopic holes in heat sink.
Place very small dab on cpu. Same procedure with the plastic bag. The top of the CPU should have just a haze. Attached heat sink to cpu.
Idle temp 32C. I haven't over clocked or stress tested it yet, to get the load temps.