I think limiting (even removing) all outbound links that are not Adsense ads is key. I have tested this numerous times over the years since Adsense came out, and each time this one simple thing shot CTR up. Every time. It isn't feasible with a lot of sites, I realize, but if you have it within your power - and if it doesn't hurt your other income sources - just eliminate or reduce outgoing links. Essentially, you're taking away one of the options for visitors to leave your site without putting some coin in your account.