Now the real work begins - promoting your book while you begin your next one. And yes, you need to do both.
Here's quote I remember, and can't remember where I heard it, but believe it's truth:
"Writing a book is thin-lipped determination.
Getting a book published is vocal persistence.
Promoting a book is screaming madness."
Yep, there it is, in a nutshell.
The reality for writers is this - once you have a name the big publishing houses will probably allow a budget for promotion (but you'll still have to promote as well).
So, the key question is, "who is going to buy this book?" It's a question you need to be clear on when you begin promoting. And, actually it would be best if you begin promoting BEFORE the book is actually published. Think about it, tweak your promotion to that audience - think focused, not shotgun.
So what can you do to promote that doesn't cost you a fortune? By all means set up book signings if you can. You won't sell a lot of books that way, but exposure is good and many readers like to feel like they get to know the writer. And remember, turn out could be very light. Don't take it personally. The bookstore may just give you a table and if you're lucky, a chair. Don't take that personally either. Bring some extra books, bring a blown up photo of yourself and your book cover, maybe some flowers and a bowl of free mints - or use your imagination and do better than that.
Don't give away any copies of your book free except to your Mom and reputable reviewers. It'll cost you a lot of money if you don't remember that advice. When you publish the publisher will generally give you a few copies included gratis in your contract - maybe 10, if you're lucky perhaps 20 copies of the book. After that you buy them from the publisher to hand out. You won't pay retail, but you will pay.
The obvious these days is social networking. Have a great following on twitter? Many friends on Facebook? Make sure you get the word out that your book is available. Give live links to where the book may be purchased. Make sure you get onto Amazon and set up your author's page, and ask friends, relatives & the folks who do many reviews on Amazon to review it.
And if it's an Ebook you've published? Advice is similar. You can arrange virtual book tours now. Amazon is a great place to pimp your book; investigate, use all the tools they supply that you can.
There's a great book out called Internet Rich, Your Blueprint to Book Sales by Mike Bray that offers a lot of ideas and resources (I take the 'rich' part with a grain of salt, but his info is excellent). And I, personally, can use all the help I can get. I can recommend Premium Promotional Tips For Writers By Jo-Anne Vandermuelen as well, a kindle book on the subject.
Dig into it, be creative and remember the promotion really rests mainly on your shoulders -- and it will always be greatly your responsibility to promote your book.