ann255 - thanks, I've really enjoyed sharing that one!
Before I think about starting anything else(mis-speaks, perhaps? I've got some great ones, all true!)
how about some sayings that aren't, as far as I know, particularly old, but deserve to live on?
"As limp as a vicar's handshake" - can mean exhausted, but anything limp, really.
"As camp as a row of tents" - effeminate etc.
"As wet as a haddock's bathing costume" - an ineffectual person.
Everlea.