Choose the best domains by reading a lot about what's out there in the world... put your finger on the pulse of the globe and form an understanding of the global zeitgeist.
Check this out:
http://www.google.com/press/zeitgeist.html
Google Zeitgeist is a nice overview of the search trends Google monitors on its system. You can get a feeling for what's happening. When world events hit, check this out and see how people react. Even general news items like the death of public figures, wars, or popular films can give you ideas. Be sure to visit the Google Zeitgeist archive too.
The world is more often than not TOO BROAD to really determine the marketability of domains that are GOOD, though not *astronomically* great. For that, look through your local newspapers, and try to spot common terms or subjects that aren't trademarks and might be available.
Don't get crazy. Before buying any domain, don't ask yourself "Can I see this as a Website". Honestly, there have been so many silly/strange websites, we can probably envision anything as a website and not bat an eye. Better to ask "Can I see others reacting favorably to this name and the idea that makes it bankable."
Simple
popularity isn't good enough these days, having some notion of how a website can convert its apeal into money will help separate good domain names from DUDS.
One nice exercise is to use an Expired Names search tool, and pitch the "word" part of the domain into Google and see what pops up. Are there lots of links to this topic? Are people wild about it? Is their large amounts of commercial activity around this type of topic?
These are general rules to apply to domains of any topical persuasion. You may want to buy Lee Hodgson's book "The Insider's Guide to Domain Name Speculation". I'm sure it will be your BEST resource for learning from those that have come before you.
Best regards,
W|Z
