If you aren't published yet, or are self-published but would like to be published by a regular royalty-paying press, I dedicate this paragraph to you:
When it comes to big-time publishers, well, I haven't gotten their attention myself, and apparently most people can't without an agent. But I've had a lovely experience with small presses who do mostly ebooks with some POD (print-on-demand), and highly recommend giving them a chance. I like this yearly poll of readers' and writers' favorite publishers for ideas on who to look into. My own publishers, The Wild Rose Press and Central Avenue Publishing (formerly called ireadiwrite Publishing) both placed very highly on it, and with good reason. I've enjoyed working with them both a great deal.
Now I'll address the authors who are newly published, either self-pubbed or published by a small (or even big) press, and are realizing they (the authors) are still expected to do a lot of the marketing. First of all, congratulations on the publication of your book! What do you do next? Here are some things that have worked for me:
Goodreads - sign yourself up as an author there. It's free, and it's a delightful site where avid readers hang out. So is Amazon, but Amazon has gotten huge, selling everything under the sun, while Goodreads doesn't sell anything and is dedicated solely to book discussions. Putting up a few free copies of your books on the "giveaways" section will attract lots of attention, at least making hundreds more people aware that you exist.
Book bloggers - search for blogs that review books similar to yours, and email them to ask if they'd like to review your book. Again it means giving away a free copy if they are interested, but the review generates a lot of exposure. And actually, Goodreads is a good place to find such bloggers: look at the reviews for books like yours, and click on the profiles attached to the more thoughtful and well-written reviews, and frequently you'll find a link to an official review blog listed there.
Spam everyone you know - well, don't actually spam them, of course. But make a list of the email addresses of everyone you can think of who might conceivably be interested in knowing about your writing news (I use an Excel spreadsheet), then send out a brief and cheerful update every once in a while. To help gather email addresses, stick a bit of code on your webpage that says something like "To be notified of my writing news, enter your email here. Your address will never be shared or sold," with a box where they can submit their address, which will then be automatically emailed to you. I had no idea how to do this, but asked an I.T.-guy friend, and he gave me a very simple code that did the trick. Thanks, Rich! Mine's here, if you want to see it.
First chapters - make them available for free on your webpage, or somewhere else easy to find. If people can read it and get hooked, they'll buy the book. And they'll always appreciate getting a feel for what the writing is like.
That ought to keep you busy a few days. Hope this helps! Best of luck to you. Now I suppose I should actually go, uh, write. But thanks for letting me procrastinate a little longer.