Try to put onl two ad units per page and the higher the better... I like to post it on top of the article after the title and one in the sidebar. Works well for us.
Well, your eCPM is basically how much Adsense estimate you will earn every 1000 PIs, so I like to think of it as a cross between your CTR and your CPC. Basically, put your ads in a better spot so that you get a better CTR (what percentage of people click your ads), and target higher paying keywords to increase your CPC (cost per click). If you can improve your CTR and CPC, then your eCPM will improve as well
Revenue = CPC x CPR x Pageviews To increase CPM, you need to increase Revenue while Pageviews remain constant. Otherwise, you have to decrease your total pageviews while making your revenue constant.