Good old fashioned sales and word of mouth.
I'm learning this the hard way. I thought the hardest part was developing the product.
I'm curious: Why did you think that? There are articles, from time-to-time, that decry this notion, and yet I find that young/first-time entrepreneurs often think this. Since it's such a common notion - I've become generally increasingly curious about how it (this idea) is spread. How it gets into people's heads. Any thoughts?