It isn't so much difficult as time-consuming:
1. Niche-oriented content -- there has to be a market for what you write about.
2. Get traffic by using good SEO, networking and building back links through article submissions.
3. Keep visitors on your pages with interesting content and media. The longer they visit, the more likely they are to click.
4. Recognize that it takes time. My own AdSense agenda is going up slowly, but it IS going up and to be honest, I don't put in a lot of time because I make my living writing for others. The average Joe or Jane can do it! I'm very much an average Jane. :)