api versioning strategies, like putting the version in the url, in the headers, or some other way

Hey everyone, I'm trying to figure out the best way to manage API versioning as our service gets bigger. We're tossing around a few ideas, like putting the version right in the URL (you know, like /v1/users), using special request headers (maybe Accept: application/vnd.myapp.v1+json), or perhaps even doing content negotiation. What's actually worked well for you all out there? Any nasty surprises with certain methods?