BE PREPARED!
Research the organization in advance and prepare a list of questions. Do not ask questions about information that you can easily find elsewhere.
Do NOT ask for a job or even help finding a job!!!
Ask for career information and advice.
Arrive early!
If meeting at the workplace you can learn a lot just watching while you are waiting and you'll make a good impression by being on time!!
Always bring copies of your resume.
Offer a copy to provide background for your contact's advice, NOT for them to help you find a job per se.
Be focused about the information you need.
Dress professionally.
You're there to make a good impression as well as gather information.
Tell the contact all of your questions in advance or at the beginning of your meeting. Then suggest that he/she address them one at a time. Generally contacts will enjoy sharing their experience and may otherwise get carried away with one question and run out of time for the others.
Maintain eye contact, smile, and nod occasionally to indicate your interest.
Ask for names of other contacts so you can expand your network. Unlike a job interview, take notes in an informational interview.
Follow up with a well-written thank you note by the end of the next day.
Questions may vary depending on where you are in the process of your investigation. Here are some very basic questions, which clearly may need to be embellished for your specific agenda.