Only describe or show a physical or particular attribute if it is peculiar to the story. Otherwise leave it to the reader to imagine his own preference.
Remember - beauty is in the eye of the beholder, and if your detailed description differs from my imagined one...
Many popular book heroes or heroines are never actually described at all. Check back.
Exactly what I was gonna say.
If the character's physical appearance affects the story somehow, by all means describe. For example, if a character is being a target for murder because she looks like the woman the villain of the story has been after for years, there's a mistake and he's after the wrong woman, by all means describe her physical appearance. If it's not that important, don't describe. I've read works from authors that don't describe their character's physical appearance at all and it still works.