A few helpful tips, such as how to speak the vocabulary of your customers, and monitor places they hang out at (author mentions Facebook, Twitter, but you could throw in specific forums, anywhere where your customers discuss and share advice, linkedin groups etc).
I think getting introductions is key too. Start with your friends/linked in contacts and ask them if they can refer to some people in <industry>. Then ask these people if they can refer other people too.
Otherwise, it's kind of difficult to cold call or cold email these people and have them agree to meet. I tried cold calling for a software product of mine (filesharing for small companies, in France) and it was quite difficult, especially considering I don't have a sales background. When you say "Mr X told me you could be interested" you get an almost-guaranteed meet.
Another way to do it if you picked an audience who's internet-savvy is to analyze conversations you find in forums, and find evidence of problems people have (that's the approach that Amy Hoy recommends in her online class 30x500), and create educational content for them: ebooks, screenscasts, podcasts, blog posts etc. as far as I understand, it's not exactly customer development though, as it doesn't get you in face of customers. Just a different approach to find potential customers :-)
Thanks a lot!
I'd add in "Use Rapportive Gmail plugin to determine email address of relevant contacts"
If you type all these when composing an email, one of them will pull up a bunch of their social networks attached to that email, while the others will do nothing. That's how you can quickly figure out someone's email by guessing + Rapportive.