There are a couple of ways in versioning your API

1. Through REST URL . domain.com/v1/Customers, domain.com/v2/Customers

2. Through HTTP Header – add the version as part of Content-type

3. Add the version on the REST object itself as one of the attribute/property

A few excellent articles:

http://stackoverflow.com/questions/389169/best-practices-for-api-versioning
http://codebetter.com/howarddierking/2012/11/09/versioning-restful-services/