As the Service Portfolio, part of the Service Model, matures, changes to the service interface, and the service contracts are inevitable. I was approached with one solution: to put them under change control. However expedient this measure, it is not a cure, but temporary stabilizers of organizational turbulence. Changes to the underlying IT capabilities that reflect the business needs that are dealt with in a proactive manner tend to stabilize over time and yield the agility that helps power sustained business performance.
Properly designing the services, i.e., proper Service identification techniques and practices are paramount for alleviating these types of issues. Second best is to use a registry and repository to store and manage them centrally. And of course you need a source code control system to manage the code that is going to be using the services.
The use of correct Service identification for alleviating the risks and impact of Service Interface and Service Contract changes coupled with management and governance in a Service Registry and Repository are thus paramount.
Further, as services abound and the organization feels more comfortable in the creation of new or modified (see above) services, the need to manage and govern this Service Proliferation Syndrome becomes increasingly important. Again there are two aspects : one of which is doable at design time and another at runtime. Service Identification coupled with Service Refactoring and Rationalization (SRR) help mitigate the first aspect -- design time best-practices. Secondly, the monitoring of Services at runtime come hand in hand with the Service identification and Monitoring approach to alleviate Service Proliferation.