If you want to keep your ecpm rate high, then you could try some of these..
1. Keep a lower number of adsense ad units on your site. Google pays the highest for the first ad unit only. After that it starts diminishing.
2. Build your site around highly paid keywords. The rates for keyword keep on changing and are decided upon the number of searches made for that particular keyword.
3. Try to get more traffic on your site. Higher the number of views, the ecpm also tends to improve..
This should hopefully help you..
