[Mitch sent me this one, which he heard in high school:]
Essays should be like a woman's skirt. Long enough to cover the subject, but short enough to keep it interesting.
[Which reminded me of another sexist but memorable maxim -- I do not know where from:]
Translations of poetry are like women. The beautiful ones are apt to be unfaithful, and the faithful ones are apt to be ugly.
[Not nice -- but entertaining advice from an earlier age. Half the fun is in the idea and execution, and half the fun is imagining the venue in which this would have been appropriate.]
[This post was slightly edited at 8:57pm]