If you've been running AdSense ads on your site for quite a while and you feel that you aren't making as much cash as you could, in 99.99% of cases you're right. Most people feel frustrated that they cannot generate the revenues they expected, and this is down to traffic levels as well as click through rates.
There are many ways to turn traffic into AdSense traffic and here is a short list of them with a description for each.
The first thing that you need to do is tune up your site's contents. While for some people this isn't an option (if you're running a forum for example it's a lot harder) most people can make their AdSense sites veritable cash cows.
To begin, you should find out what your site's keywords are. This is a basic technique in SEO (search engine optimization) known as keyword density verification. A good tool for this job is SEO Density Analizer.
You should then take this list of words and search the Overture Search Inventory or Google AdWords Sandbox to find out what other words could be doing better then what you currently have.
From that point on try to keep your site focused on the things that appear mostly in the keyword suggestions you receive. Try to include those keywords, or at least some related keywords as often as you can in the links on your site.
Because of the way Google AdSense works this is a sure way to increase your earnings because you'll get better ads in your AdSense banners, similar to how you would get a better Google PageRank
It's also important that you continuously add new pages to your site. This is very important, as the more pages you have, the higher the chance is that you'll get more hits on your ads.
It's also very important that you choose the appropriate formats for your AdSense banners and the best locations for them. While this is a much wider subject you should generally know that the top three Google AdSense formats are: 336x280 large rectangle 300x250 medium rectangle and 160x600 skyscraper.
Make sure you choose the appropriate colors for the ads, as making them too different from your site's content (or just different in a bad way) is sure to make visitors regard them as banners which have nothing to do with the site itself.
Make sure these ads are positioned where they'll generate the highest amount of profit. Most times, that location is said to be right before the beginning of your site's actual content.
On pages with a long text body, you might find skyscrapers to do better, simply because users get exposed to them for more time. Also, for sites with news or similar items, you might find a good position to be at the bottom of your content, because that's when people are finished reading your story and are looking for something else to do.
You should use more AdSense units if you have a lot of text on your page. Just make sure not to overdo it because you might find that you will get a lot lower CTR and lower revenues then you were getting in the first place by employing this technique inappropriately. You can add as many as three units, so use them wisely.
There's a Google AdSense Preview Tool that will let you visualize what ads would get served on your page and it can come in very handy as a testing mechanism.
Just make sure you respect the AdSense policy regarding click fraud. As a pointer you could place a picture next to your adds, just make sure to leave some room in between in order to make sure this isn't regarded as encouraging visitors to click the link.
Of course there are more ways to increase AdSense traffic from your site, but these should serve as a good starting point for income maximization.