Do not treat one-way calls as asynchronous calls.

Do not treat one-way calls as concurrent calls.

Expect exceptions out of a one-way operation.

Enable reliability even on one-way calls. Use of ordered delivery is optional for one-way calls.

Avoid one-way operations on a sessionful service. If used, make it the terminating operation:

[ServiceContract(SessionMode = SessionMode.Required)]
interface IOrderManager
{
   [OperationContract]
   void SetCustomerId(int customerId);

   [OperationContract(IsInitiating = false)]
   void AddItem(int itemId);

   [OperationContract(IsInitiating = false)]
   decimal GetTotal( );

   [OperationContract(IsOneWay = true,IsInitiating = false,IsTerminating =
true)]
   void ProcessOrders( );
}

Name the callback contract on the service side after the service contract suffixed by Callback:

interface IMyContractCallback
{...}
[ServiceContract(CallbackContract = typeof(IMyContractCallback))]
interface IMyContract
{...}

Strive to mark callback operations as one-way.

Use callback contracts for callbacks only.

Avoid mixing regular callbacks and events on the same callback contract.

Event operations should be well-designed:

Avoid using raw callback contracts for event management, and prefer using the publish-subscribe framework.

Always provide explicit methods for callback setup and teardown:

[ServiceContract(CallbackContract = typeof(IMyContractCallback))]
interface IMyContract
{
   [OperationContract]
   void DoSomething( );

   [OperationContract]
   void Connect( );

   [OperationContract]
   void Disconnect( );
}
interface IMyContractCallback
{...}

Use the type-safe DuplexClientBase<T,C> instead of DuplexClientBase<T>.

Use the type-safe DuplexChannelFactory<T,C> instead of DuplexChannelFactory<T>.

When debugging or in intranet deployment of callbacks over WSDualHttpBinding, use the CallbackBaseAddressBehaviorAttribute with CallbackPort set to 0:

[CallbackBaseAddressBehavior(CallbackPort = 0)]
class MyClient : IMyContractCallback
{...}