eCPM is the contrast of your impressions to your total earnings. An example: You've recieved 250 impressions, and from those impressions a few clicks, which totalled to $5.00 in earnings. Since CPM is Cost per 1000-impressions, this means that the eCPM will be $20. Your eCPM rises according to your impressions and total daily earnings.
eCPM is determined by two factors CTR & CPC The key to increase your CTR is add placements, add appearance and new visitors. Add placements can do wonders to increase your CTR. Although there are general guidelines (top, left, etc), you need to experiment on your site what works best Add colours. Most people would say the better the add colours blend in with the content the better. A few people claim they have better success with bright colours that change often. Again, you need to experiment to see what works best. on average your CTR drops when you have the same visitors come back every day as they get used to the adds and stay clear of them. Getting new visitors is key to keep CTR up. The key to increase your CPC is you site keywords, quality of adds, and add placements. There are no hard facts on CPC for keywords as this is a closely guarded secret by Google. However by signing up as an advertiser you can compare keywords and focus on higher paying keywords Add quality, again lots of rumours but not a lot of facts as Google keeps this close to its chest. Finally add placement. The highest paying adds will display in the adds that are loaded first on your page. So make sure you have the highest CTR on those. If the adds that load first have a low CTR, remove them and you might actually make more money.