There is an art to choosing keywords, but it's a science as well. When you are relying on ad revenue, you want your content to match closely with ads that people will be interested in. To do that, you need keywords that are relevant to ads that people will want to pursue. But, if those keywords aren't popular enough, your intended audience won't get to the article in enough numbers to make the article lucrative. The answer then is to choose keywords that interest people in learning more about the topic and that bring lots of people to the Internet to search.
The Google AdWords Keyword Tool is a great resource for finding popular keywords. It tells you not only how popular each keyword is with Google searchers but also how popular it is with advertisers. There's a lot of other data there, including how much the ads are sold for, and that's an important factor in choosing the most lucrative topics and keywords. But, when you want to write a quick article, using Wordtracker is a quick way to find popular keyword phrases. Some of my most popular eHow articles have come from Wordtracker searches.
Sometimes when I'm in searching mode I'll plug in a topic that I'm interested in and take a look at the most popular phrases that come up. Then, I start several eHow articles as drafts, each with one of the popular phrases in the title. The drafts will stay there as long as you like, so you can come back and write the actual articles when it's convenient. If you have plenty of drafts built up, you will have plenty of article titles right there in your account to choose from. Anytime you want to write, just open a draft and begin.
eHow articles allow up to five keywords, so use them wisely. You don't have to optimize your articles to use all five keywords, but a good article can easily utilize the three to five that the template asks for without making the keywords sound forced. Using your phrases two or three times in your article is enough to bring in people who are searching for it.