One trick that has worked for me is to offer the audience, or people who want to make requests, a songlist to choose from. That way they will always pick songs that I know instead of stammering around asking for songs I don't know (endlessly). I can't play everything and wouldn't want to play it if I did so guiding the audience to where I want them to go seems to not only save us both frustration but it adds to my tip jar as well.