When it comes to requests, and you will get them, an old friend who still saws a great bluegrass fiddle at age 87 provided me with a great tip. He has a ragged piece of paper that he carries with him at all times. The titles of a couple dozen songs are written on the page in blue-ink pen, red-ink pen, black-ink pen and pencil. Some are written, some are printed, some are scrawled. When someone requests a song, he merely points to the sheet of paper and says "I'll be more than happy to do the song just as soon as I get all these other requests out of the way first."
As for the wannabe singers, I merely tell them I only have one mic--my headset mic, and I do not allow anyone else to use it. As Don stated, the wannabe singers can really make you look bad.
BTW: I learned a lot from Don while on my whirlwind, 4,200 mile trip to the sunny south. Anytime he gets to rambling you should be listening carefully and taking notes. His playing skills are pure magic.
------------------ Travlin' Easy