Very rarely do I take the time to go into detail about what the characters are wearing because, as has been stated, for the most part, people know what these characters are wearing. And so far I've only had clothing disappear on me once... and it was only a pair of panties and that was totally intentional (Ahh... gotta love HP fics... and vanishing charms ).
Anyway, If I am writing about a night out on the town, or something where the characters wouldn't be in the normal everyday kinds of clothing, I will take the time to be a bit more discriptive, and then, as has also been stated, I keep it as realistic as possible. Some characters aren't going to go out in a custom tailored suit, while others wouldn't' be caught dead in jeans. That is part of the character.