Google trends helped me a lot with a very simple optimization that increased my website’s traffic (and my adsense income!) with 1/3.
My astrotest website (in dutch only, I’m working on an english version) used to draw quite some traffic on the search term “sterrebeeld”, meaning “starsign”.
But in dutch this word is commonly spelled in 2 ways, the other way being “sterrenbeeld” with an additional ‘n’ in the middle. This is due to a recent spelling reform, but I won’t go into that topic…
With a Google trends comparison I found out that ‘sterrenbeeld’ was far more common as a search term than ‘sterrebeeld':
Thus the very simple optimization of replacing “sterrebeeld” with the much more popular “sterrenbeeld” on every page of my website (and waiting for Google to reindex the site) increased my traffic and my income!
It can be worth checking that you are using the most popular spelling of important keywords (or even use both)!