Pick out a specific section of your book and upload it as a separate title as a freebie, making it very clear it’s an initial installment – particularly useful if you have only one title. However, it’s got to satisfy, and be worthwhile as a stand-alone – or should have a really tempting cliff-hanger at the end – so that the freebie may entice readers to go for the rest of the novel once they’ve read the first bit. @debbieflint
Run a Contest. I’ve seen some authors create contests like sending in photos, or videos, or stories. You pick a winner and give a prize. This type of activity can also work before the book comes out. @Beth_Barany
More tips, including running giveaways (e.g. don’t make them too short!) can be found on Marking for Authors blog.