I have done some "Informational interviews" with prospective employers before when I was on the job market. It's basically like a reverse interview, where you ask the questions. But it gets your foot in the door with people who otherwise might never actually see your resume, and can really work for getting job offers. Here's the basic plan:
1 - Do your research. Find out as much as you can about the company that you are interested in. Find out who there top management people are. Find out about their history and mission statement. Find out what positions they have open. Find out anything that you think might be relevant.
2 - Write out your questions. You want to have about 5 questions (less than 10 for sure). They should cover the breadth of your interest in the company. They should be short. They should tell you whether this company is a place where you want to work. They should be personalized to the company and demonstrate that you are specifically interested in them.
3 - Climb as high as you can get. When you call to set up the "informational interview", try to get it with as high up a person in the company as you can. Be friendly. Be open about who you are. And don't ask for much. Introduce yourself and say that you have looked into their company and are interested in them for "x" reason (based on #1 above). Then tell them that you would like to meet with them to learn more about their company. Tell them that you only need about 15 mins, and ask if there's a time that you could meet that week.
3a - If they push to hard against meeting, then you could ask if there's someone else that you could meet with instead. Or you could just ask your questions right there on the phone. Doing it on the phone is definitely not preferable. You will get information, but you won't get any benefit on their side of things.
4 - Show up for the meeting dressed well, but not too formal. You should appear relaxed as you are interviewing them, not the other way around. Be about 10 mins early. And be friendly to the secretary. That is the person who is likely to be weeding out resumes when people apply for jobs. It would be nice if they put yours on top
Bring your questions written out, and a pen and notepad to take notes on their answers. Again, be friendly (smile, eye contact, firm handshake, etc.) Ask your questions and keep the interview moving. Make sure that you can be done in 15 mins. At that point, if they are interested in you, they may ask you some questions, and keep you there longer, but if not, then you should be ready to go.
5 - Thank them for their time. Thank the secretary. Go home. The next day you should send an email thanking them again for their time and telling them how much you enjoyed talking with them.