The answer is simple: sales and marketing.
If you have a specific question on this sales or marketing, go ahead and ask it. But if your question just boils down to "How do I get more customers?" you are in the same boat as every single entrepreneur in the world. And if the answer was simple and able to be found on an internet website, you'd have no business advantage. The difference between whether you succeed or fail is whether you can answer that questions yourself by educating yourself and executing on your idea.
Ideas are a dime a dozen. Execution is the key. And there are no shortcuts.
Well, if you built a product and you don't have customers yet - probably you wasted a lot of time/money.
The reason for me saying that is that you should constantly talk to your prospective customers while you're building your product. If you did not do that - there's a great chance that you built something that nobody wants.
The premise and question is somewhat awkward/backwards. The better scenario is to find customers FIRST, then build the product.
But, of course, this is not a perfect world and many of us have built a neato, whiz bang product and are now looking for customers.
I realize Michael Pryor is not asking for himself and that this Q/A exercise is for creating THE ONE answer, but it is still worth noting that the order of
is not the only way to do things.
In addition to what Michael said, I also like this guy - http://www.softwarebyrob.com/ and his book (Stay Small). He's very down to earth and practical.
Oh, and on Mixergy.com you can find some really good tutorials, especially this one: http://mixergy.com/free-first-sections-pr-on-a-budget-course/