Back to basics. From the research that I have done, people who search for things on the web, search by business type and location. This is no different for searches for lawyers or law firms. I review the web reports of my clients and see a lot of searches that my look like "DWI lawyer Texas" or "bankruptcy attorney Albany".
Having a great website with relevant content and good SEO will help your firm's website capture some of these searchers.
There is also a free and easy way to capture these potential clients through Yahoo and Google local listings.
For Google, go to: http://www.google.com/local/add and add your firm. You can add your website, address, phone number, hours of operation, types of law practice, what payments your firm accepts etc. You can even upload a logo. The whole process takes no more than 5 minutes. Below is a sample search "family lawyer San Antonio" I did on Google. You can see all of the local listings of law firms at the top of the search results! This trumps organic search engine marketing and is absolutely free.
The same can be done on Yahoo! at http://listings.local.yahoo.com/csubmit/index.php
This is free, and local.