I have some experience in this area. When I moved to Ireland, there were a lot of folk clubs and I was, soon, working once or twice a week at Slatterey's on Capel St. I, later, found out that I had been reccomended to the manager by some of the musicians I had met at the Sunday sessions at O'Donague's. Working gave me a little money and enough exposure to get jobs ay other venues. It helped that I was American and, thus, different from other acts, but I never had to ask for a gig because I had been touted by well know, respected performers.
In Scotland, I did a wonderful three weeks tour, booked by folk DJ, Arthur Argo. He didn't know me from Adam but he had been called by Dr. Ken Goldstein (the great folklorist), who asked him to handle me.
It didn't hurt that Jean Redpath was staying with the Argos when I showed up. I had met and sung with Jeanie at the Fox Hollow Festival, that summer.
It is the same story in the USA. Bookers listen to people they admire or respect. They, often, listen to performers who do well in their venues. Ms Lemon says that a friend was able to get a booking in a club she would like to play. Here's what she should do, after her freind does the date, have the freind introduce and recomend her. This is the only, sure fire meathod I know for getting jobs.
It works on every level. Two years ago, I was visiting a freind in a rehab hospital. I saw, on the bulletin board, that a pianist, with whom I am familiar, was performing for the residents, that day. I stopped off to say hello and he took me to the activity director and told her how good I was with that kind of audience. Six months later, the activity director called and hired me for a performance. She hired me back, this year, too. Once again, it was personal reccomendation that started the ball rolling.