Key take aways:

• Make the required part of your contract as small as possible
• Prefer optional members over required, so as not to break your clients
• Use multiple endpoints to differentiate versions
• Communication with consumers is key
  • Consider a public wiki or RSS feed for consumers

     Recommended reading:

• Emerging Guidance: Versioning Web Services, Don Smith
• Programming WCF Services, Second Edition
• Web Service Contract Design and Versioning for SOA