How to maintain the custom methods while using generated classes using RIA services and entity models?

How to maintain the custom methods while using generated classes using RIA
services and entity models?

How to maintain the custom methods while using generated classes using RIA
services and entity models ?
Everytime the model changes, the only way to generate methods for these
entities is to regenerate the DomainService class. But if we do that, the
custom methods that we have created in the DomainService class gets
overwritten. Is there a better way to do this?