get some Arctic silver 5, its the best thermal paste..
Quote from their site
"ONLY Arctic Silver thermal compound should be between the processor core and the heatsink. Remove any thermal pads or other interface material from the heatsink before applying the Arctic Silver. Thermal pads can be scraped off with a plastic tool that will not scratch the bottom then the remnants can be removed with a xylene based cleaner, (Goof Off and some carburetor cleaners) acetone, mineral spirits, or high-purity isopropyl alcohol. "
I use Isopropyl alohol, works a charm. Read the instructions before you apply, you'll find you only need to use a very small amount of the paste supplied in the syringe.
Also blow any dust out of the heatsink fins.