Don't forget your solder connections if you are really trying to reduce your resistance. Standard electronics solder is usually 63% Tin and 37% lead (or thereabouts) and doesn't make the lowest resistance connection available. With the government trying to get rid of lead in everything (it is, after all, a deadly poison) there are silver solders popping up everywhere. I have been using the 96% tin and 4% silver stuff you can get at Radio Shack. It doesn't flow as well but it make a higher strength and lower resistance connection (if only by a few microOhms).
By the way, if you are attaching the heatsink permanently (IE: it will NEVER need to be removed) you could try JB Weld - it conducts heat way better than those thermal tapes. Just don't get it on anything you don't want to be electrically conductive. If you'd prefer something more 'elegant' than JB Weld then Loctite makes a line of thermally conductive epoxies that work quite well - here is a link to one example:
http://www.contacteast.com/product/g...arent_id=11487 They make several compounds for different applications.