Something you might want to try is to put a few bills in the case, (some ones, twos and fives, along with some silver). Lets 'em know any cash is accepted. We used to start the hat like that in basket houses in the 60s. Gotta give 'em the right idea. Look professional. Shine the axe (guitar), and pay attention to your audience. It's a business or it ain't. Mostly, if you can hold 'em for a song they'll leave you something. Some days will be better than others. No shades--people like to see eyes. Get a set of songs that seem to work. Upbeat is good; these people are on their lunch hour, and they may be hungry. They might give you three minutes, but you won't get ten. What songs are you doing now? What's the age of your audience? How long you been playing/singing for?