Getting new clients is a bit different form keeping clients that will last longer. If you do a great job with your existing clients, they will refer you others (not to mention they will stay with you for as long as they keep getting results and they can afford it). The length of period someone will stay with a trainer it depends on the ability of that trainer to deliver results. If the results are not what they clients have paid for, then no matter how many new clients come thru the front door they will not stick around.
In addition to the other suggestions, I would look for opportunities to get involved in community events to build clientele. For example, local races or health fairs can get your name out to meet potential clients.
In addition, any new client has the potential to be a long-term client for you, it just depends on many factors. You have the opportunity to showcase yourself during those first few sessions and display your knowledge, compassion, motivational skills, etc. Those clients that feel value in your training are more likely to be retained. It may take some time, but you will build a following–and your niche in this industry.
Find an area in training that you are passionate about, and love what you do–that will be a magnet for you.