What makes something timeless?

Times constantly change, and there's no way to stop it. I've been constatly warned of placing popular culture into my novels because by the time I've finished writing that so-called fad is over, so how old would it be when it finally gets published?

The same can be said about choosing the stories to write. The one big tip out there is, as writers, write what you want rather than what is currently selling, because it might not be that case when you finish the story and need to find an agent/publisher (I'm having that problem with my vampire novel, and I've been working on it before the fad became huge).

So how can a writer choose what audience they write for when things may be different when the story is complete? I've been noticing these differences when I watch older movies and tv shows (70s and 80s). It's definitely obvious that times are definitely different from 20+ years ago.

I guess the answer is, well for me, to write what I want to write and worry about the audience part when it comes to selling the story. But, I suppose not using obscure pop culture references would also help.

So, what  makes a story timeless? How do you handle writing for the generation gap? Or, isn't it a problem for you when you write?