If you've bought one of my books, then thank you very much, you've done PLENTY. But if you liked it and want to go the extra mile, pleasepleaseplease post a review on the site where you bought it.
Why? Well, one advantage of self-publishing on sites like Amazon and Barnes & Noble is that they are actually quite good at marketing titles to people, and I of course have no marketing department. Let's say that you (like me) are a fan of Lois McMaster Bujold's Vorkosigan Saga. (How big a fan am I? I spelled all that right on the first try! And I can't spell!) And let's say you've bought her books on Amazon, and then you leave a favorable review for Trang there. The next time someone goes to buy a Vorkosigan book, Amazon will recommend to them that they take a look at Trang as well.
How effective is that? It can work pretty darned well! According to this article in The Wall Street Journal (which you probably need a subscription to access, argh), Karen McQuestion, who has never been published by a traditional house, has sold 36,000 copies of her books in less than a year this way!