EDIT: Oops, Sikerdebaard replied with exactly the same advice while I was typing my reply ^^

How much thermal paste are you using? If you use too much, it'll act as a thermal insulator rather than a conductor.

When I apply paste to my CPU's, I put a tiny spot about the size of a matchhead on the CPU die, and then use the straight edge of a plastic card to spread it out into a thin smooth layer over the whole surface.

For Core 2 CPU's, the metal cap on the CPU is quite large, so you'll need more than a matchhead sized dollop, but it still needs to be smoothed out into a thin layer.

This has always worked well for me.