If that happens, many times you'll find yourself not working at that establishment anymore. You want to emphasize that people who enjoy karaoke, usually stick around for hours. You should put together a portfolio of all the work you've done. My portfolio has samples of flyers I've done to advertise my shows at other establishments. I like running contests.
Contests usually get BIG crowds to come in.
This usually makes dollar signs appear in the eyes of the person I'm dealing with. Over time you'll find what works best for you. When I'm negotiating a deal I TRY to get the person to sign a contract. You may want to contact an attorney and have a general contract drawn up that you can use for each job you book. You also want to confirm how and when you'll be paid.