a small mustard seed size of thermal paste on the middle of the CPU is good enough (http://images.pcworld.com/images/article/2011/12/amd-tim-6653410.jpg). Don't put too much, or it'll do more bad than good.
Make sure both CPU and heatsink are very clean and free of dust particles or oil coating. Don't spread the thermal paste over the CPU, let the heatsink spread it by itself.