//Server interface
[ServiceContract(CallbackContract = typeof(IGUIService))]
public interface IServerService
{
    [OperationContract(IsOneWay = true)]
    void Subscribe();

    [OperationContract(IsOneWay = true)]
    void Unsubscribe();

    ...

}

//Client interface
[ServiceContract]
public interface IGUIService
{
    [OperationContract(IsOneWay = true)]
    void MessageCallback(string Message);
}

[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class ServerService : IServerService
{
    internal event SettingsUpdatedHandler SettingsUpdated;
    internal delegate void SettingsUpdatedHandler(ServerService adminPort, EventArgs e);

    private static List<IGUIService> Clients = new List<IGUIService>();

    public void Subscribe()
    {
            IGUIService callback = OperationContext.Current.GetCallbackChannel<IGUIService>();
            if(!Clients.Contains(callback))
                Clients.Add(callback);
    }

    public void Unsubscribe()
    {
            IGUIService callback = OperationContext.Current.GetCallbackChannel<IGUIService>();
            if (Clients.Contains(callback))
                Clients.Remove(callback);
    }
}

internal void SendMessage(string message)
    {
            for(int i=Clients.Count - 1; i >= 0; i--)
            {
                IGUIService callback = Clients[i];
                if (((ICommunicationObject)callback).State == CommunicationState.Opened)
                    callback.MessageCallback(message);
                else
                    Clients.RemoveAt(i);
            }
    }