In this post Fredrik Normén concentrates on configuring the WCF service in RIA Services.
Have the following in mind when creating your DomainService:
There is a wire between the server and the client, so be careful and not pass too much data over the wire. In a enterprise application, an app with a lot of users or low bandwidth it’s advisable to create a light weight class (Data Transfer Object) instead of instead of just passing a DAL type generated from Linq to SQL, Entity Framework or a domain entity. Only pass the data the View needs, this will reduce the number on data passed over the wire.