I think the big "secret" lies in the development of the site. Before making a site to "earn a few bucks", consider everything you can about that site and it's content. Has someone already done this? Chances are, they have. Take a look at a similar site and find out what is working on that site and what could be done better. How are they getting recurring visitors? The best traffic by far is search engine traffic, that is a given. Think about your site on a sidewide scale, not one page at a time. Only SEO's for large companies need worry about the small stuff such as keywords and keyword phrases. If you spend the time developing unique and
helpful content, you'll notice a number of search terms being used to locate your site without doing
any SEO work! Build relevant back links, and get your site out there! Sorry, this post is sounding more like a marketing consultation than an adsense strategy. I am sort of going off on a tangent here, so back to adsense. If you are looking to make good money, I think there are many more practical ways that take a fraction of the time over using adsense. If there is already a wealth of information on a topic out there, don't expect to capitalize on the same thing. Instead, do us all a favor and develop a site that makes something easier, or provides a unique service. Only then you will notice the real secret.
Also I want to thank rob6188 for his input, he really has some useful information.
EDIT: Ack! I posted this before I read GoPC's post which is pretty much what mine says (only better). As far as strategies go, I noticed a considerable jump in CTR after placing a link unit under my header image. Each advertiser pays differently per click. With that said, if you are looking to make good money, cosmetic surgery. These companies are willing to pay the most because they make the most! It's simple.
Examples:
www.breastenhance.info - Site recieves around 25 visits per day. NO unique content. Makes $1.50+ per click. (~$7 a week right now).
www.yadadamean.com - Site recieves around 150 visits per day. Tons of unique content. High CTR, LOWEST possible payouts. Has gotten 20 clicks today and has made $.60.
There was a list somewhere of highest paying keywords I used to have bookmarked. I am going to find and post up here.
-Dylan