When I think of promotion, I think of one word: Visibility.
Other than a great product with a good word-of-mouth, I really believe it’s all about visibility. There are a number of ways to do that. Some are listed below.
Offer your book for sale or free: For indie authors, there are a number of sites to help promote your book for a fee. A few of the sites are: Pixel of Ink, Book Bub, The Cheap and The Fugal Ereader.
Facebook: Be as active as you can but don’t bury people, and don’t hit people over the head with your book. They’ll get tired of it. Posting funny quotes and pictures that people share helps with name recognition.
Facebook has an option for advertising on their site by pay per click. You only pay if someone clicks on your ad. Some people have had success with this, but you have to do your research to narrow down your demographic, etc. Also you can create an event on Facebook and invite your Facebook friends.
Twitter: There are many groups out there where you can join, or you can create your own group where you can retweet your groups tweets. Make them fun and interesting. You have only a number of characters to catch a person’s attention. Hootsuite is a great site to schedule and keep track of your tweets. I highly recommend them and they’re free!
Goodreads giveaways: You can offer a book giveaway on Goodreads. I’ve done this a number of times, and it does create more visibility. When I do this I've always managed to get my book on a number of friend's 'to-be-read' list.
Goodreads events: Create an event (book signing, sale day) and invite your Goodreads friends. Also limit the amount of events you create. Like anything else, they can become very much like spam and turn your friends off.
Book reviews: Reviews help boost your visibility. Goodreads is a great place for getting reviews. Many of the reviewers there have their own review sites. Also I have noticed a correlation with reviews and sales. If I receive a number of favorable reviews on Goodreads in a given week, I do see a jump in my sales.
Newsletter: I almost forgot one of the most important opportunities available: growing a list of people for your newsletter and actually sending that newsletter out. You can grow your list through a number of opportunities, one being contests. Have someone signup for your newsletter in order for them to be entered in a contest you are having. I seriously think this is the most important promotional tool you can use.
There are so many promotional opportunities available for writers, it's hard to decide and at times confusing. The key, I believe comes down to focusing your energy on one or two and following through with them.