Mihaela Lica on the Everything PR blog provides a service by listing/linking to seven online directories of public relations firms. You probably want to be in at least some of them, for you never know when a prospective client will turn to a directory for help in finding a firm for a given relational need.
The seven directories listed are O'Dwyer's PR firms database, Council of Public Relations Firms, PRSA, PR.com, Public Relations Institute of Australia, Canadian Public Relations Society, European Public Relations Confederation, and German Public Relations Association.
Word of mouth, networking, and inventive use of social media are probably the best ways for smaller PR firms to become known, but it doesn't hurt to be listed among the "biggies" either. Think about a directory presence.