This sounds like one of those studies that make you go "Duh," but a team of scientists has apparently put substantial research into this burning question.
Here's part of the abstract of Cornell University Prof. Michael Lynn's study "Determinants and Consequences of Female Attractiveness and Sexiness: Realistic Tests with Restaurant Waitresses," published in "Archives of Sexual Behavior." I am not making this up.
Waitresses completed an on-line survey about their physical characteristics, self-perceived attractiveness and sexiness, and average tips. The waitresses’ self-rated physical attractiveness increased with their breast sizes and decreased with their ages, waist-to-hip ratios, and body sizes. Similar effects were observed on self-rated sexiness, with the exception of age, which varied with self-rated sexiness in a negative, quadratic relationship rather than a linear one. Moreover, the waitresses’ tips varied with age in a negative, quadratic relationship, increased with breast size, increased with having blond hair, and decreased with body size. These findings, which are discussed from an evolutionary perspective, make several contributions to the literature on female physical attractiveness.
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Link for more. You'll have to pay to read the full article, though.
http://www.springerlink.com/content/d5w770x766q28444/