WRITE a review or at least leave a rating for the book. You may not know this, but a lot of promo sites won't let authors promote their books unless they have a certain number of reviews. It also helps others to decide if they want to try the book.
LIKE it. When you see something about the book, click the Like button. Definitely LIKE the author's page.
TAG it and share the link. It's awesome if you take a moment to share the link to the book so that others might discover it and purchase the book themselves.
People are busy, and I understand you can't do this for every book you read. But if you like the author and you're impatiently waiting for them to finish a series or just waiting for their next book, do this stuff for them. They'll appreciate it, and it might help them have more time to write. Less time spent promoting means we have more time to finish our books.