Snappy, memorable and trend creating: hashtags aren't just for Twitter anymore.
Creating a hashtag for your book makes it easy for fans to talk about your book and find each other. It also makes it easier for you to keep track of what readers are saying and engage with them. That's a heavy load from a free tool.
When deciding on a hashtag you need to keep a few things in mind. First, hashtags are still mostly used on twitter where posters are limited to 140 characters. You want to keep your hashtag short so it doesn't cut into their character limit too much. Second, unless you have a short title, you'll probably need to use something else, but don't stray too far. Your hashtag needs to be easily associated with your book. Third, make it memorable and distinguishable. For example #Amysbook is generic, has little to do with your book and can easily be confused for any other Amy's book.
You can use your hashtag anywhere and everywhere. I've even scene hashtags tucked into the corner of a book cover. For sure, you should include it on your website, especially on the page featuring your book. Include it in your online bio pages, too. I also recommend asking anyone else who posts about your book to use the hashtag. This can include reviewers, tour hosts, cover reveal hosts, and whenever you do a guest post. You can even tuck it at the bottom of your press releases.
Like all aspects of marketing, the more you put into it the better. If you want to see a great example of author interaction using hashtags, jump on twitter and check out #icons. Margaret Stohl keeps her fans engaged by retweeting thoughtful comments and posting updates and thanks, all utilizing the hashtag. On the day of release, there was no where on twitter that wasn't covered with #icons.
For today's task, select a hashtag for your book. Do a quick check on twitter to see if anyone else is actively using it. If not, run with it everywhere you can.