API Versioning Strategies: How Do You Roll?

I've been thinking about API versioning recently. You know, like putting a 'v1' in the URI (v1/users) or using headers (Accept: application/vnd.myapp.v1+json). What have you guys seen work well, and what hasn't, out in the real world? Am I missing any other common ways to do this? And how do you handle keeping old stuff working while adding new features, so you don't break anything for people already using your API? I'd love to hear what you've tried and what you like best.