also, just like any other self-employed job, you gotta be hungrier than the other guys in your field. While they sleep, you write up a marketing plan. While they watch sitcoms at night, you design a catchy logo. While they're spenging money at Outback Steak House, you take that cash and buy a few more shirts. This goes for all personal service fields, not just PT.
Above all, know your shit. If you keep up with current trends and have a solid base of knowledge and experience in your area, you'll make a living on it. Just be ready for the next "trend", whether it be core balls, super-slow, or if HIT makes another round. If you get in front of it (or help define it!) you'll make some decent coin.