Successful book launches begin long before a book is released. They start in the conception process, when the idea first arrives. Authors who ask questions about the identity and needs of their audience BEFORE beginning to write are more likely to craft a message that resonates with readers.
During the writing process, social media can be used to generate excitement for your book in progress. It may feel a bit uncomfortable to talk about something that isn't yet complete, but a side-effect of these discussions is feeling an added pressure to finish what you've started. I've been involved in more than one writing project that I wouldn't have finished if I hadn't previously announced the release date.
Getting people involved in your writing project is one way to engage your audience. Find beta readers who are willing to give you feedback on your manuscript before it goes to print. Make your book available for pre-order. Post a chapter or two on your blog to promote interest. Tell the story of how you came up with the idea for your book, and talk about your writing journey.
If you are releasing an e-book, consider giving your book away free on its launch date to quickly build interest. If you've created a quality product, the money you lose by giving your book away free will be quickly made up (and then some) as people discover your work. There are a number of sites that allow you to post the dates your e-book will be available for free, and many of them don't charge for this service.
When the day of your book launch arrives, communicate prolifically! Post links, content snippets and videos, run ads, and send an announcement to your email list.
Once your book has launched, keep talking about it so it isn't forgotten. A sustained marketing campaign will be far more effective than an initial push with no follow-up.