As in all advertising, the more a business is listed, the more it is noticed. Another bit to note is that some professional sites will create a web page for your listing. This is a great resource if you do not have a web page but would like to place more information online. Once you have it created, link to it from other listings as well. Most professionals belong to various organizations.
Many professional organizations are online.
If you find that an organization you belong to is online, check to see if they list members. Professional Community Finally, link to others in your professional community. Professionals make partnerships all the time. Place a link on your site to them and they can return the favor in kind. For example, an attorney who only handles Patent Law may link to a peer who handles Criminal Law or an attorney may link to a private investigator they use and vise versa.