Look for smart women who were not physically attractive as children / teenagers, but have become attractive adults.
Hot teenage girls can get cocky and become accustomed to coasting on their looks to earn attention from men.
Nerdy and unattractive teenagers must be unusually witty or entertaining to earn attention from men. If they become attractive adults, they retain the braininess cultivated in youth, with the added plus of physical attractiveness.
I am not sure whether the same theory could apply to women searching for men.
Bottom Line: Dating web sites should include two photo boxes: a recent photo and a photo from when you were 15 years-old.
(Thanks to Maria P. for helping generate this theory. Bottom Line is partially in jest.)