Towards the top of the thread, Anahata wrote:-
Make a demo CD.

Personally, I have become a bit wary of demos. There was the guy I booked a few years ago and when, at the end of the evening, I said that his guitar style on his demo was different (and by that I actually meant much worse), he admitted that it was another guitarist on the recording, but nowhere on the demo was this stated.

Then there are the number of young performers who are fine singers and musicians but when you see them live turn out to be "shoe gazers" with no audience communication skills.

Seeing performers live is the answer. If I am booked at a festival, I never forget that I am also a club organiser and try to see as many other acts as possible, however heavy my schedule is; my record was over 90 a few years ago at Whitby. It's not just checking out people that you have not seen before. Previously, you may have caught someone on an off day; many young performers are improving all the time; some established acts end up resting on their laurels and just coast along. For me, it is very important that the people that I book are talented, sincere about their music but also able to put it over well.