Your book is your product, and you've got to let people know it's coming just like the big studios let people know a new movie is coming by teasing them with a trailer.
Choose whatever strategy works for you, and read the Goodreads blog for great case studies and examples.
However, don't kill yourself or your bank account.
Do what you can, but start doing it at least 6 months in advance.
If you're on Twitter, tweet about it.
If you're on Facebook, post about it.
If you have a blog, blog about it.
If you're on Patreon, let your patrons know about it.
When you meet people in person, tell them about it.
This will do two things.
It'll make you believe that the book will happen.
It'll make everyone ask you questions, and so motivate you, and get excited with you, and want to snatch your book as soon as it's published.