Define a behavior in your .config
file:
<configuration>
<system.serviceModel>
<behaviors>
<serviceBehaviors>
<behavior name="debug">
<serviceDebug includeExceptionDetailInFaults="true" />
</behavior>
</serviceBehaviors>
</behaviors>
...
</system.serviceModel>
</configuration>
Then apply the behavior to your service along these lines:
<configuration>
<system.serviceModel>
...
<services>
<service name="MyServiceName" behaviorConfiguration="debug" />
</services>
</system.serviceModel>
</configuration>
You can also set it programmatically. See this question.