Our local downtown merchants and the local "Friends Of Music" association used to combine to seed our case with $100 if we promised to play for 2 hours in the location we were assigned. They also provided us with a sandwich board to give us some legitemacy (sp?). The next year it dropped to $50, and after that we got the sandwich board.
I found that a guitar, no matter how well it was played, wasn't too effective at attracting attention. When I switched to an autoharp, it was different enough to interest people. I put a sign in the top of my case that said,"Broken String Fund - Visa or Mastercard cheerfully accepted". A lot of folks chuckled at the sign and I never had anyone ask me to accept Visa or Mastercard.
I also kept a playlist taped to the back of the Autoharp and put a small dot beside the tunes that seemed to earn me the most $. Since no one ever stayed around for more than 4 or 5 songs, it was all right to repeat songs.