Informational interviews are information-gathering sessions,usually focused on a job or career field you're interested in. They give you an opportunity to get answers about what a typical day is like, what the person likes or dislikes about the field (which is what you may like or dislike about it), what it takes to enter that field, and what it takes to be successful in it. In good informational interviews, you may even get advice on your situation and your best career/job search moves. Informational interviews are strictly for you to get the "inside scoop" from someone who knows. (FYI: If you're lucky, you might get a job lead, but it's very bad form to go into the interview expecting this person to help you get a job. If you're actively job hunting, check out my Hidden Jobs Finder. It will show you how to use LinkedIn and other tools to contact hiring managers who will have job openings for you.)
If you need an informational interview, it's probably because you are new to an area--which means you probably don't have anyone to ask to speak with you. So if you can't get an informational interview by going through your current contacts, how do you get it?
Use LinkedIn. Once you create a profile, you can make connections and introduce yourself to people on LinkedIn, and then ask them directly for an informational interview. Most people are flattered to be asked, and won't mind talking to you for 20 minutes. If they're really pressed for time, they might offer to answer questions by email--which you should definitely follow through on. Also, you can join groups and participate in discussions, and post your questions there. This can be an especially effective tactic for entry-level job seekers. I've seen some really great LinkedIn discussions positively packed with valuable information for job seekers.
LinkedIn pages are tremendous sources of information on people you'd like to interview and companies you're interested in. Once you've set up your interview, use LinkedIn to prepare for it just as thoroughly as you would for a job interview. Get all your ducks in a row so that you don't waste that person's time by asking questions you can look up the answers to. Coming to the interview prepared with background knowledge and intelligent questions leaves them with a great impression of you as a confident, competent go-getter they will remember (in case they run across a job opportunity for you later).
After the interview, remember to send a thank you letter. If you can, include a relevant article or a solution to a company problem--something helpful to them. Then, include them in your network by routinely contacting them every few months. A successful informational interview gains you valuable information and an expanded professional network--and who knows where that might lead?
(If you don't have a LinkedIn profile that gets you contacted by recruiters and hiring managers about jobs, use my LinkedIn Profile Tutorial. It will show you all kinds of things you can't learn from LinkedIn about how to attract hiring managers like a magnet.)