I always found "honesty to be the best policy." My own Modus Operandi in responding to requests that I do not know is to simply say: "I'm sorry but I don't know that song."

Of course, I'm not that blunt. I temper the statement, and appease the requester by adding on something like "hey, that's a really great song though....I'm going to get the music to that and.......LEARN IT! This way the person is honored for his musical knowledge and it takes the edge off of the simple "I'm sorry but I don't know that song."

My other response is to say something humorous like: "you know, when I first started playing music there were only two songs in the world: Moon River and Hello, Dolly. Now 50 years down the line there are hundreds of thousands of great songs out there, but, unfortunately, you can't learn them all. And, even if you could, you'd forget them fast not being able to play them regularly." People like to hear odds and ends snippets like that when said in a humorous way.

Mark