Avoiding Duplicate Boilerplate Code in WCF Service

I'm responsible for maintaining a web service project in c#. I have one service class with a bunch of methods that look a lot like this:

    [OperationContract]
    [WebInvoke(Method = "POST", UriTemplate = "v1/Login", ResponseFormat = WebMessageFormat.Json)]
    public string Login()
    {
        try
        {
            JObject incomingRequestJson = retrieveJson();
            JObject returningJson = new JObject();

            string accountId = incomingRequestJson.SelectToken("accountId", true).ToString();
            string username = incomingRequestJson.SelectToken("userName", true).ToString();
            string password = incomingRequestJson.SelectToken("password", true).ToString();

            LoginResult result = readerBLL.Login(accountId, username, password);
            returningJson.Add(new JProperty("userSessionId", result.userSessionId));
            returningJson.Add(new JProperty("friendlyUserName", result.friendlyUserName));

            return returningJson.ToString(Newtonsoft.Json.Formatting.None);
        }
        catch (Exception ex)
        {
            throw new WebFaultException<string>(ex.Message.ToString(), HttpStatusCode.OK);
        }
    }

Note that retrieveJson() grabs a JSON string from the HttpContext and runs JObject.Parse() on it. Note also that the consumer of this service is a Flex app, so I do need the WebFaultException business with the "OK" response code (The networking component in Flex treats connection timeouts and all HTTP error codes exactly the same - no further data other than

I don't like the way I have the try-catch block repeated in every method, so I tried making the following method:

private object invokeBusinessLogic(Func<JObject, object> f)
    {
        try
        {
            return f(retrieveJson());
        }
        catch (Exception ex)
        {
            WebFaultException<string> wfe = new WebFaultException<string>(ex.Message.ToString(), HttpStatusCode.OK);
            throw wfe;
        }
    }

Then my service methods can look like this:

public object Login()
{
    LoginResult result = (LoginResult) invokeBusinessLogic(readerBLL.Login);
    returningJson.Add(new JProperty("userSessionId", result.userSessionId));
    returningJson.Add(new JProperty("friendlyUserName", result.friendlyUserName));

    return returningJson.ToString(Newtonsoft.Json.Formatting.None);
}

However, this has the problem that the use of JSON to carry the request data gets exposed to the business layer. Is there a better way to avoid the duplicate try-catch blocks than this invokeBusinessLogic idea?