Coach Glassman has a very good article in this month's CrossFit journal titled, "on being a trainer." Definately worth checking out.
Not much else to add other than 3 P's of Personal Training: 1.) passion 2.) personality and 3.) purple-ness
Passion - not much to say about this one other than if you don't love what you do your chances of success of very much lower.
Personality - this is perhaps the most overlooked skill a personal trainer can possess. When I worked out at Gold's Gym while in college, I cringed everytime I watched one of their miserable, doopey, muscle-bound trainers give a 60 min session with that look on their face like "I'm just too cool, tough and big for this job" as they walk between machines flexing their muscles in the mirror. Whatever you do, don't be one of these guys. Instead show you care, be personable, and get to know your clients on a personal level. If you have a good personality, love what you do and deliver results the marketing will take care of itself.
Purple-ness - This is a term coined by marketing guru Seth Godin describing
Cows, which after you've seen them for a while, are boring. They may be well-bred cows, Six Sigma cows, cows lit by a beautiful light, but they are still boring. A Purple Cow, though: Now, that would really stand out. The essence of the Purple Cow -- the reason it would shine among a crowd of perfectly competent, even undeniably excellent cows -- is that it would be remarkable. Something remarkable is worth talking about, worth paying attention to. Boring stuff quickly becomes invisible.
This goes along the same lines of what Mike suggested about finding a niche. Narrow down your speciality into a smaller market (say sports performance) and be as bold as possible. In other words, don't settle for the ordinary.
When you pour your heart into something you love doing, things will fall into place. Do it!
100,000 generations of humans have been hunters and gatherers; 500 generations have been agriculturalists; ten have lived in the industrial age; and only one has been exposed to the world of computers.