Certainly the follow-up contact to your initial letter should be by phone. You might want to reiterate something discussed that was particularly helpful or informative. In addition, maintain a record of names, dates, comments and referrals for future reference. Keep these people posted on your progress. Write or call them periodically to let them know what you are doing.
First impressions count heavily.
Be on time, shake hands firmly, smile, look professional and maintain good eye contact. Ask answerable open-ended questions that start with words like how, what, why and in what way. Ask impromptu questions. Do not make the mistake of using your informational interview as a pretext for getting a job. It will not work and it can be embarrassing when it becomes obvious.