[sdphil]

the answer if quite simple. Once you have the knowledge down ( we all assume you'd have that) then self employment comes down to proper servicing of your clients. You need to develop a business plan that your customers feel that can not be easily had elsewhere.
I guess in the photo bus it would be. Creativity, pricing and rapport with people involved as well as professionalism.
I don't know about a photography business but I own a handy man service and I can say even though I am not cheap my regular clients don't even consider going elsewhere.

As for making a living in photography, I think as in the music business I have heard many BAD musicians that are very successful and many great musicians that do other things to make a living.
I have also see some what I consider to be very uninspiring images from very well known photographers that make tons of money taking pictures.

I did a shot of a friend that was blown out and overexposed, the sun was behind her and I tried to fill her face with flash and opened the lens for shallow depth of field. The entire upper background was nothing but white and there was little detail in the lower part. I was able to adjust the exposure PP and bring back some color and detail to her face.
Technically, the shot was trash but it did have a "look" to it. My friend LOVED it and I am sure she would have paid for it by her response to it.

So, we live in a free market especially when it come to art. I don't think anyone can tell anyone they don't have what it takes to make money being a photographer
If you can find a client that is willing to pay you then your in business.