This is really quick, and really simple.
A good way to promote your book: Use a Goodreads giveaway. Like this one I’m running for Dangers Untold. (If the fancy widget doesn’t show up for you, use this link here. Giveaway ends October 8th.)
A bad way to promote your book:
Spam your twitter followers. Or worse, like this clown, thank them for following you while spamming them, and when they’re not even following you. Kind of a giveaway.