If you don't have a publication date (or a publisher), then start marketing your author PERSONA. Get your author website up and running; start a blog (if blogging is your thing); create an author page on Facebook; or post a few of your PERSONA's favorite things on Pinterest. Start to build a web presence for your author PERSONA, BEFORE you get the book deal.
We've chatted with agents and editors during writing conferences. Most admit that if they like the pages you submit, one of the first things they will do is a web search for your name (or pen name) to investigate your web presence. Why do they do a web search? Well, they want to know how committed you are to your writing career. If you aren't actively promoting a PERSONA online, they may question your ability to market your books. (Stay tuned for future posts on how to market your PERSONA online.)
WARNING: unless you plan on self-publishing your book, don't post any preliminary or working titles, mock covers, or other artwork for your manuscripts. If you do post these items, readers may search for your work only to be frustrated when they learn it's not available for purchase. Or worse - some unscrupulous writer, and there are more than a few, may 'borrow' your ideas, copy your cover, or otherwise steal your thunder.
If you do have a publisher and a publication date....as our critique partner, Mercer Addison, would say, "Getta cracka-lackin'!" Approach your target market / readers in their natural habitat and start getting the word out that your book is coming!