Consider signing-up for AdWords and create a mini campaign. It can help you to understand the working of AdSense.
Target a specific audience. Get Google AdWords.

Put your targeted-keyword on Page Title.

Repeat the keywords in your content.

Learn HTML.

Put emphasis around your keywords by using HTML tags like <h1>…</h1>, <h2>…</h2>, <strong>…</strong>, <b>…</b>, e.g. <strong>AdSense Tips</strong> .

Get domain name that contains your targeted keyword.

Submit your website to directories for inclusion, e.g. dmoz.org, yahoo directories

Submit your URL to search engines for crawling.

Create and submit your sitemaps to Google Sitemap (sitemap.xml) and Yahoo (urllist.txt)

Google is not the only search engine. Optimize for different engines as well. You are very likely to face less competition than in Google.

Invest for original fresh content. Write or pay for contents regularly.

Use copyscape.com to find content theft. You invest for your content. Don’t let it be taken by unscrupulous webmasters.

Article marketing is one of the best way to deliver traffic to your site. Write and submit articles to article-submission sites. Include short summary and hyperlinks to your website in the author information box.

Don’t use objectionable methods to draw visitors to your site; buying traffics, spyware, hidden-texts, page cloaking etc will get your AdSense account terminated.

New visitors have higher chance to click on the ads than regular visitors, thus higher CTR (click-through rate).

Regular visitors have higher chance to recommend your site to others.

Search engines are where most of your new visitors come from. Learn a bit about Search Engine Optimization, or pay people to do it. Doing it yourself will save yourself from troubles created by not-so-honorable SEO (Search Engine Optimizer).

Don’t create mirrors. These are sites with different URLs but same contents. It hurts search engine ranking.

Its easier to create many websites with low earning than few websites with high earning.

Forum generally have high impressions count, but very low CTR.