Pop Culture References in Writing

Want to make a reference to your character with a scar like Harry Potter, or a smile like Jennifer Aniston? When can we use them? In high school creative writing classes, the rule is generally: never.

But. What if it's really good? What if it's really funny? What if it's an important part of the character or the way people interact with them (a kid who looks like Harry Potter is bound to get a few snide comments now and again)?

Here are some guidelines:

• Does it really offer anything? Move the story forward? Shape the character? If it doesn't have a purpose, it shouldn't be there. 
• Will it be recognizable in ten years (or, as long as you expect the piece to be around)? Classic icons are usually a better choice than modern sensations. If a reference is too obscure, it's more likely to alienate a reader than anything else. 
• If you choose to use a reference to describe or compare someone, don't put too much weight on it. For example, "his white hair stuck out in every direction, resembling Albert Einstein" is better than "he looked like Einstein" because the second one requires the reader to know who Einstein is to get the image the author is trying to convey, while in the first one, if you've never heard of (or seen a picture of) Einstein, you're not thrown out of the story. 
• Know you're audience. If you're writing a short story for a teen music magazine, references to modern pop-stars are much more acceptable than they would be in a sports magazine.

How do you feel about authors referencing other works of fiction or celebrities in their work? Have you come across any that worked? That didn't?