I've managed to get some elusive €0.01 cent clicks recently. I find a combination of [exact match], correct landing page, and high CTR attribute to this most.Awww, thanks you guys.
One little thing I forgot to mention in that post is that minimum required CTR is lower for different match types. I'll say no more.
As for a resource, I have read the following blog three times over:
Random Musings about making $$$ on the net.
This guy knows his stuff and each post has some really golden information.