We use WCFExtras (http://www.codeplex.com/WCFExtras) for that.
Among other features it allows live exporting of your code xml comments into the generated WSDL, for example check how these xml comments:
///
/// Retrieve the tickets information for the specified order
///
/// Order ID
/// Tickets data
[OperationContract]
TicketsDto GetTickets(int orderId);
get reflected in the WSDL of that interface:
Retrieve the tickets information for the specified order Order ID Tickets data
An excerpt from their docs:
Adding WSDL Documentation from Source Code XML Comments
This extension allows you to add WSDL documentation (annotaiton) directly from XML comments in your source file. These comments will be published as part of the WSDL and are available for WSDL tools that know how to take advantage of them (e.g. Apache Axis wsdl2java and others). Release 2.0 also includes a client side WSDL importer that will turn those WSDL comments to XML comments in the generated proxy code.