How do you handle API versioning when your services are always changing?

We're setting up some new microservices and need a solid plan for versioning our APIs. We've looked at a few ways to do it, like putting the version in the URL (v1/users), using the header (Accept: application/vnd.myapi.v1+json), or adding a query parameter (users?version=1). What works best, really, especially when we have to make changes that break things and want to make it easy for people using our services to switch over?