Actually I think it depends more on the value of your users to the AdWords advertisers. If you are sending them "good" traffic (like they are converting and buying stuff), Google recognizes that. If you are sending through clicks that ultimately have very little value to the advertiser, they will adjust accordingly.
Thats good to hear that there are acutally factors that can increase it. I noticed a substancial increase in my CPC rates after I started sending adsense 80% of my traffic rather than 30%