You can try to place some filters on which ads appear I'm not sure how flexible this is since I've never bothered with this myself but by and large this is out of your control. Payout is determined by many things including search trends, basic supply and demand, the weather, whether or not the sky is blue.... There are just so many factors involved and publishers have very little control over practically all of them. One day there could be 20 advertisers for your keyword, the next day there could be 10 and all of a sudden there's less competition and your payout bottoms out. Then the next day Dr Oz could mention a product you are promoting on his TV show and the next thing you know you're making $4 a click instead of .50 cents.
The best things you can do are build your sites around keywords that have high adwords competition, have content that is laser targeted to your advertisers, chose a KW that has historically large payouts, and a strong past history in Google trends, and then ensure proper ad placement/type. Avoid banner ads, text ads only. The top left of the page is the "hottest" area for clicks. Make the ads seem like a natural part of the "feel" of the site. In other words, make the ad seem not like an ad but within the bounds of the Adsense TOS.
