Informational interviews are excellent opportunities for you to learn about different career fields. You are gathering information, asking advice, and learning about a career field or major. Remember, you are not asking for a job.
When contacting a new person, don't put them on the defensive by asking them for a job or a recommendation. If you do that they will become an "Assessor," who will begin assessing whether or not you are worth recommending. Instead, simply ask them to share their story. Let them talk about themselves rather than listen to you pitch your skills. That way you are building a relationship (an "Ally"), and they may even advocate for you.
Organize your questions ahead of your interview. Try to avoid asking questions that can easily be researched online.
Be sure to ask if they are willing to keep in touch, and if so, how to reach them in the future? If they are willing to stay in touch, be sure to follow up. Also, after your interview, be sure to send a thank you, either card or email.