When you make that first instantaneous judgment about whether or not you like how someone looks, you're actually being influenced by a whole slew of things you don't realize you find attractive. At first glance, the basics of attractiveness seem pretty straightforward - humans like health and youth because they like reproductive fitness. Generally, humans also like sexual dimorphism (men with strong jaws and large brows; women with large eyes and youthful features). None of that should be too surprising. Humans are pretty predictable in this respect, and they really aren't much different from the lowliest bug who desperately wants to mate with whichever other bug would enable it to procreate the best.
That's how it seems at first glance, anyway. Beneath the obvious stuff, a lot of other cues determine whether you like what you see when you look at someone's face, and we waste no time in making that determination. It takes about a tenth of a second for you to see a face and jump to a whole bunch of conclusions - whether the person is trustworthy or not, aggressive or not, and, obviously, whether he or she is attractive. But if you were to write down a list of the things that you assume you find attractive, and then read this list, you will probably not find much overlap. From the smell of someone's sweat to the amount of vegetables a person eats, there are tons of features we're attracted to without knowing it.
This one isn't about having an athletic body shape; although that sort of thing is obviously attractive (you probably don't need a study to tell you that muscular men have more sex, but here's one for you anyway). Rather, this is about how we can subconsciously determine the healthiness of someone's diet from his or her skin. One study found that a diet high in carotenoids from fruit and vegetable sources resulted in a complexion that was rated as healthier and more attractive. Counterintuitively, another study even found that consumption of garlic, which has numerous health benefits, made a person's body odor smell more pleasing to sexual partners in the long run.
It might sound crazy, but each person's immune system is totally unique and has the capability to assess the similarities and differences it has to the immune systems of those around it. The "major histocompatibility complex" (MHC) is the thing your immune system uses to distinguish its own cells from other cells. Everyone's is unique. Studies have found that humans are attracted to people whose MHC is most unlike their own - or, at least, they prefer how they smell. One study had male students sleep in t-shirts for two nights and then asked female students to rate the shirts' scents; the women most preferred the smell of men who had the most dissimilar MHCs. A separate study reached the same conclusions in reverse - men preferred the scent of women who had immune systems unlike their own.
Static images don't tell the whole story when it comes to attractiveness; we actually glean a great deal of information from how a person moves (e.g. men prefer the gait of women who are ovulating). One less obvious example is that women tend to make a lot of small, slow movements when they're interacting with a man they like, and men, in turn, tend to find this attractive without necessarily noticing it. Another study found that men tended to be more attracted to women who made slower and less-complex body motions. Related research has determined that slow movements and slow speech are generally more effective at winning over people who are still somewhat cautious around you - for example, on a first date.