Just focus on writing quality content that attracts repeat visitors, really. Especially with the economy in the pits, you have very little control over what advertisers are spending on AdWords these days. It really just comes down to common sense: follow good SEO practices, publish new content daily, and work on building quality backlinks to your sites. You can try experimenting with different ad sizes and colors to see what works best for your visitors, as long as you do this slowly and are sure to get enough impressions with each test to make the data meaningful. Good luck with your sites!
There's no best way. You can do alot of things like -writing on topics that have a high payout per click -increase your CTR by changing ad positions and colors.
eCPM = 1000 X CPC X CTR Therefore, there are two way to improve your eCPM. Either improve your CPC or CTR