When does your brand become immortal?

The other day on Damn, I Wish I Thought of That, Andy Sernovitz asked:

When does your brand become immortal? When customers will pay cash to wear your logo.

Sundaes is the best ice cream joint in my hometown. They do a hell of business on word-of-mouth alone.

And when I was in high school, it was actually cool to wear a Sundaes t-shirt like the ones the staff wear. They didn’t advertise. They didn’t even have a marketing department.

My question for you: How do you get there?