I am a folk/acoustic presenter and receive hundreds of demos a year. (Sadly, the majority of these aren't very good). What I don't want is a studio production. I want to hear what your act will sound like on my stage. What I want to know is how you will sustain the interest of my audience for two hours.
Send the material and follow up with a phone call in 3-4 weeks. If I (or any other presenter) say I can't use you, believe it and don't persist.   
I listen to every submission (mostly in my car) even if I zap through it after 10 seconds per track. As a volunteer, I don't have the money or time to make long distance calls to tell you you don't suck too badly but we can't use you.   
Send a "live" CD (tapes tell me you haven't invested in yourself). Send me the lyrics. If I like the sound I WILL spend the time to try to understand what you are saying.
Duos, trios and more have a better chance of getting a booking because of the harmony/instrumentational variations.
If you are a solo act with a guitar, you better sing well, play fingerstyle well and be able to "entertain" (i.e. "amuse") in between songs. You need a repertoire of two hours of "A" material and some good stories to tell while you a replacing a broken string.      
There are literally thousands of "guys with guitars" out there and it is tough to pick out one among the many.
If you are a songwriter and only perform your own material, tell me a story. I'm not interested in "I, me, mine" songs (younger acts are most guilty of this).
Jeb, I hope this helps you to understand the other side of the equasion.