Relevance and consistency help the engine rankings. Web Strategies addresses all of these items in trying to optimize search engine listings. A popular site often gets higher rankings, and some engines give preference to sites that have other sites linking to it. See if your trade association or other organizations offer opportunities to link. Get the word out any ways you can.
Advertising is an option, although it can require a major investment.
Banner exchanges are one inexpensive way to start. There are also some reciprocal programs, where you get placement on random sites and accept random banners on yours.