While you're building a client base, you'll likely be doing things like free consultations, free group training workouts, cold-calling new members, greeting members at the front desk, free body-fat assessments, handing out samples, etc. Anything you can do to get to know potential clients really. At this point, you have to think of yourself as the ambassador of the company.
Once you're well-established, and you have a full schedule it's just a matter of keeping clients. This means being able to maintain a super-positive affect for each and every one of your clients, giving them great workouts, and not letting a long day show on your face.