I recently learned a few tips about an informational interview. First of all, come to the interview on time and prepared. The person you are meeting with is doing you a favor. Secondly, don't bring your resume or ask about a job. You are there to gather information about how they got where they are and to get advice for someone interested in working in their field. Moreover, while you are "interviewing" them, they are also interviewing you. Treat their time with respect and try not to be too informal. At the end of the conversation, ask them for a few names that they recommend you connect with and if they mind if you use their name when you contact those individuals. Here are some other interesting articles about informational interviews.
Informational Interview 101
Informational Interview Pet Peeves