I've never used a standard signature line but always liked the line from the Terminator movie series: "Come with me if you want to live" - it makes me smile.
A few times I've used a funny quote for my signature.
Jeanette Isabelle
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I'm not sure whose twisted idea it was to put hundreds of adolescents in underfunded schools run by people whose dreams were crushed years ago, but I admire the sadism. -- Wednesday Adams, Wednesday