You must define what separates your site from others in the industry. You must also know exactly what your audience is looking for and give it to them quickly and easily. Leverage related sites and services in your industry. Partnerships can really work.
For example, if you sold automobile headlights, I would exchange links with car clubs, repair sites, do it yourself car sites...you get the picture. Also, advertising in some targeted, high circulation newsletters may help quite a bit. Focus on servicing the customers you have and keeping them...word of mouth will spread and you will gain a good reputation.