A number of sources have released articles on male and female perceptions of sexual attractiveness in nonverbal body language and more specifically facial expressions
PDF of article from University of British Columbia on body language and perceived attractiveness
Statistical analysis of facial expression from OkTrends
Statistical analysis of perception of attractiveness in opposite sex and response to perceived attractiveness
The basic gist of these findings are that men and women have distinct differences in deciding on sexual attractiveness when it comes to body language. While the common belief has been that smiling is the best way of appearing attractive, both the study from UBC and the statistics from OKTrends say otherwise.
For men, looking either prideful and triumphant or swaggering and brooding had a significantly positive effect on perceived attractiveness over a friendly smile.
For women a smile was still better then neutral and is comparatively perceived more attractive in women then men. A flirtatious face was the more attractive then a smile though.
The effects of eye contact where split between genders. Men on average benefited from not making eye contact in profile pictures on dating sites. Women benefited from the exact opposite. Combinations of eye contact with different facial expressions returned mixed results. Giving flirting signals with out making eye contact was over all the least beneficial combination and received on average the lowest responses.