1) The first hard and fast rule is to make sure the person who wants to be an influencer has something to offer in exchange for that free book (afterall, you have a limited supply of those freebies!). If the person is a good friend but they have no interest in doing more than patting you on the head and telling you what a good writer they are, they won't make a good influencer.
2) Find someone who is willing to spread the word via a well-established blog. Offer to do a written or video interview for them.
3) An influencer should be willing to post reviews on Amazon, Barnes and Noble, Goodreads, CBD, Shelfari, and maybe even offer some new places you've never considered before.
4) Local book clubs are a great source for finding influencers. Start with your local library for a list of book clubs in your area, then choose those groups whose interest most closely match your genre. Offer to speak at the club and/or do a Q&A.
5) If you're close friends with someone who has a natural network that relates to the theme or genre of your book, use that connection. For example, a historical romance set during the Civil War might be of interest to museum stores. This way you are tapping in to word-of-mouth sales on a subject already of interest to the potential buyer. Talk about ready-made fan base!
6) Social Media the title! Influencers who use social media to Tweet, FB, Linkedin, Pinterest (or whatever source they most use to help spread the word) will help boost the visibility of your title.
7) Influencers might even be willing to go to their local bookstores and encourage the manager to stock your title.