The answers to this question are all very valuable and provide a number of actionable suggestions. However, I have to disagree that these lead generation activities are FREE. They are not because they are very TIME consuming and a person has to be hired to execute all suggestions above.
The first step for a chiropractor, an expert in the field, is to clearly define what is the VALUE clients/patients would get from a visit. Segment people into different groups based on their needs and speak to their problem and how you can solve it. Personally, I went to a chiropractor after a recommendation from my Ob/Gyn when I was 7 months pregnant and in serious back pain. Weekly visit for spine adjustment took away the pain and I could not only walk, but even run in my last trimester. Now, that's priceless!

Once a chiropractor has clearly articulated benefits of the treatment, he or she can hire a marketing professional to execute on the strategies people mentioned on this thread.