Going through some weekend phonemail when I came to one where the caller gave her name and then said, "You don't know me from a load of hay."
(Actually, I do. The caller has been in The Slice a couple of times.)
But I liked that expression. I'm thinking I might start using it when identifying myself.
"Hi, this is Paul Turner calling from The Spokesman-Review. You don't know me from a load of hay but ..."