Currently I structure my URLs like this:

    /v1/2017-06-08/blah

The idea is that you write in the date that you wrote the implementation against the API. This is just like Stripe's API, but explicit rather than implicit. Then everything under that will continue to work as-is.

There's a few reasons I like to do this, but the main one is that I often struggle to decide whether a given change is really "worthy" of a major version change.

The only rule/caveat we have is: within a given client's implementation, you should only use one date at a time. Mixing them creates a small possibility of compatibility issues.

The initial /v1/ is just in case I decide to move away from the date thing at some stage.