Once you build a product that solves a problem customers are willing to pay for, figure out where they are and go there. At the risk of sounding trite, like pg's advice, do things that don't scale: http://paulgraham.com/ds.htm. Literally go to potential customers in person and talk to them.
- try putting your self in the shoes of your potential customer(what are they trying to accomplish?)
- start with training, consulting, coaching your potential customers.
- Promote a freebie. try making it viral and/or habitual
- hang out where your customers are.