Use a branded thermal compound and a high quality one, it's best to spend a little extra to get something that will last for a long time as compared to a standard quality compound.
Try the Arctic Cooler MX series or the Cooler Master compounds.
http://www.arctic.ac/eu_en/products/cooling/thermal-compound.html
http://www.amazon.co.uk/Cooler-Master-IC-Essential-Performance-Compound/dp/B007W81HKU
The Cooler Master link I gave you is to a very good compound that you may like highly.
How to put it on
Put a pea size amount, place the heat sink (or whatever you need) onto it so it will spread out it self.
Generally the heat will dissipate from the middle outwards so the whole plate won't need to be pasted.