Do not create sites for adsense only
If you create sites with the sole aim of making money from adsense, you will not do well on the whole with few exceptions. The problem with createing sites for adsense often leads to sites that might be described as low quality with people unlikelty to return or link to your site. If it was not earning from adsense, it might not earn from anything else. Whereas if you create quality sites, not only will it earn from adsense, you can monetise it in other ways. This may not hold true for all sites, but there is far greater chance of doing well in monetising a site that's built for the user and not merely thinking about keywords and search terms in regards to how much you will be found on the search engines and what CPC you might get with adsense.
To be honest, I was guilty of this when I first started out in webmastering a few years ago, but over the last couple of years I have realised to do well in this field I would need to maintain high quality sites so they can earn revenue various ways and not just from adsense.
I must stress I still have a couple of sites I would still would call adsense sites, but I have spent quite a bit of money redisigning one of the the sites and and added high quality content as well as one or two other features to strive to become less 'adsensy', if I may call it that. This will hopefully give people more of a reason to link to the site naturally, whcih is very very important if you wnat to maintain good rankings on Google.