The informational interview is really just a form of field research. The term was coined by Richard Bolles, author of What Color is Your Parachute (and lots of other career books). Ideally, you'll be conducting your informational interviews in person, but you'll often find yourself using the same techniques on the phone or by email. This conversation should be pretty short - 20 minutes is a good general target. Much more than that and you'll be asking too much of your contact... which brings me to the next point...
During the informational interview, you're not asking for any favors. Not outright, and not right now, anyway. This is the time that you're asking people about something they have expertise in - most people are happy to talk about something they know well and have been successful at, so you don't want to throw off that comfortable conversation feeling by appearing too eager. Of course, you're not only giving them a chance to talk, you're getting to learn about a topic from an insider, and you're building your network for down the road.
The best way to introduce yourself to your contact is by showing that you already know (at least a quick Google search worth of) something about the contact. It could be as simple as saying, "My colleague recommended I contact you regarding..." or "I read an article you wrote on...". And then one of the next things you say should make it clear that you're not asking for a job. One of the things you are asking for at this point, will be directions to the next step. There was a great New York Times Shifting Careers blog about it a few years ago with a list of questions you might want to ask during your conversation. You could ask your contact who else would be good to speak with, if there are any books you should read on the industry, what professional organizations are active in the field, how he or she got into his or her position, etc. (Oh, and you should absolutely be taking notes during all of this!) You're not only talking about what's great about this type of work - you're also addressing the difficult parts. By the time you're done, you should have a solid list that can serve as an outline for the next steps you'll take. If you still think this might be the right path for you, you'll have a good idea of who to contact next. You might find out that it's not exactly what you imagined, but there's another related job that could be more up your alley, so you can also ask your contact about that.
After your conversations, be sure to send quick thank you notes to your contacts. This will not be the last they hear from you, though! About a month after your informational interview, you should send your contact another thank you note - this one will be a bit longer, and will tell your contact what you did following your conversation. You might say that you went on to talk to other industry experts your contact had suggested, you might say you went to an industry meeting or job fair, you might say you read a recommended book or article. Not only does this second thank you note show your contact that you really did value his or her advice and that you're really committed to this pursuit, it also gives your contact a great opportunity to give you more contacts or tell you about anything that has come up in the last month. And it's further solidifying your connection by keeping you fresh in your contact's mind. Next time a job does open up, you could be the one they call!