Choosing a popular and profitable theme for your blog will determine whether or not you are able to stay in business for an extended period of time. While narrowing down the theme from a list of themes can be a fairly scientific process, choosing the themes on that list to begin with is considerably more experential.

If you're planning to market your own products, you may want to start by finding a need, rather than looking at data. For instance, do you know of a pressing need that many people have? Could you provide some sort of substantive solution to that need? If so, would a blog be the appropriate means though which to provide that solution? Something might come to mind immediately, but if it doesn't, you can proceed with data-analysis as a means of choosing a popular and profitable theme for your blog.

You can start by using a program like WordTracker to begin choosing a popular and profitable theme. WordTracker will allow you to view a list of the 100 or 1000 most searched topics on the Internet.

You can then break this list down into smaller pieces. For instance, if number 10 on the list is cars and you have some sort of an interest in cars, you can then take the keyphrase cars and search for it on Wordtracker using overture pay-per-click results and demand/supply analysis called KEI.

This will tell you two things: it will tell you approximately how hard it is to penetrate your niche; and it will tell you roughly how much money people are making in your niche. For instance, if pay-per-click amounts are high and have a lot of bid competition, that means that people are making a lot of money in your niche, which is why they are purchasing advertising for large sums of money per click. It also means that you can potentially make a lot of money for serving ads for these advertisers via Adsense and YPN.

And there you have it: through data-analysis and need-analysis, you can begin the process of choosing a popular and profitable theme for your blog. If the topic you select is popular, has a good KEI rating, and has a high PPC rating, you know you have a winner.