Problogger.net has a great article about this, which you can read most of the details at. To summarise, here are the points that are mentioned.

Topic popularity
Choose a topic that people are searching for. For example, new services, products or trends can be of benefit, as interest in them will be high at the start. Be the first to blog about it, and you're taking advantage of its initial popularity.

Topic competition and narrow niches
Choose a topic which has very little or no competition, where there's no one else writing about it, or if there are, they're of poor quality. Make sure you're better than them.

Availability of revenue streams
Ok, so you have a topic that people are searching for, and there's not much competition. Now you can make money from it. Google Adsense should be used for a high paying topic, to give you the greatest return. There are also affiliate programs on the net that you can use and are likely to give you a good return.

Availability of content
Research the topic before you start, to see if you can create a lot of content for it over a long period of time. You don't want to run out of content after a few weeks or months.

Measure your energy, passion and interest
While there might be plenty of content to draw upon for the topic you've come up with, it's important to appreciate how much energy and commitment this requires. Will you still be energised by this in 3 months time? How about in a year? Only pursue what you are passionate about. That's the key.

Pull it all together
It's rare to find a topic that everything above comes together on. Most blogs fail in one or more of these areas. It doesn't mean your blog won't be viable or profitable, but it's good to be aware of any weaknesses.

As a final note... make it happen. It really is up to you.