I never had a good client base, so take my advice with a grain of salt.
I'm inclined to second Jrahien's recommendation to specialise. Simplest way to specialise is to take the areas of fitness that you're already interested in and have personal experience with. I assume you have experience with at least one area of fitness, and that you didn't decide to become a PT first and then decide to start exercising afterwards. My areas of expertise are strength training, musculoskeletal correction (although I'm no substitute for a skilled physiotherapist and don't diagnose anything), and body composition management. I know a lot about these areas because I have a lot of personal experience and interest in these areas, and have studied them a LOT. They're also interrelated, which is helpful. Study, and, as the saying goes, do your time under the bar (practice, practice, practice and learn through experience).
You can get certified in something, but that doesn't mean you specialise in it. You can specialise in something without being certified in it (but make sure that what you're doing is legal/within your professional scope -- like Jrahien said, don't go telling people that you can be their dietician if it's not true). In 2011 I spent a weekend getting certified as a powerlifting coach, but a weekend-long course does not make an expert. The other day I was looking at the footage from the course, and realising just how incompetent I was throughout it. It takes much more than a course to become competence.
Whatever area/s you specialise in, spend plenty of time in the gym (assuming that's your working environment), giving good customer service. I suspect that one of the best things someone can do for themselves as a PT is to work on the front desk, signing people in. All you have to do is smile, greet them, and scan their membership ID, and be nice when they leave at the end of their session. PT's get clients by getting to know potential clients. On the gym floor, it can be a lot of work to break the ice with a decent amount of members. On the front desk, however, you're literally greeting hundreds of people each day. If they then see you out on the gym floor, helping people out and delivering solid PT sessions, and if they're interested in PT, they're far more likely to approach you, which is a rare situation for a PT in a commercial gym. Even if they never approach you for PT (most won't), if you then get to talking to them about what they're up to in the gym and what they're trying to achieve, you can then say things "I can help you with that," and since you've already built rapport with them, they might actually listen to you.
And that's the thing. Would-be clients will hire (and stick with) a PT who demonstrates competence, and establishes trust and rapport. There are many things you can do that contribute to this mix, but at the base level, that's it. There are sales jocks who are great at getting clients to sign up through the hard sell. But those clients will also tend to quit after a few weeks, because they can't trust someone who manipulates them into parting with their money.
