I think, if you want a character to look "attractive", just go for the fashion of the time.
Like, other characters in the story may be attractive, but they'll just be on the prettier side of the plain-Jane spectrum: they're cute, but they wear pretty average-looking clothes and average-looking hairstyles.
Your "attractive"-attractive character, however, should look like they just stepped out of a fashion catalog. They wear the clothes that everyone wants, the hair that everyone strives for. If that won't work in the setting, they should at least be as conventionally attractive as possible.
For example, in anime: in a world of mostly brunettes with shoulder-length hair, the "attractive"-attractive girl usually has luxurious, long black hair (milky-white skin, too), because in Japanese society that is what "beautiful girl" is supposed to look like.
In cultures that are less racially-homogeneous, that's a little tougher...but each race has its own beauty archetypes, too. With Caucasians it's usually the long-legged blondes; with African-Americans it's usually someone with relaxed hair and killer curves.
I realize I'm only talking about females, but that's because those examples are more vivid to me. ^^;