New Post: Store Principals between WebService calls

Hello,
when im calling:

using (var client = new ServiceReference1.BugNetServicesSoapClient("BugNetServicesSoap"))
        {                        
            var login = client.LogIn("admin", "password");            
            var result = client.GetCategories("1");
        }

LogIn is returning true, but when im trying to get categories it's throwing me null exception on Session["IsAuthenticated"] caused by not storing Princiapls from LogIn. Can you tell me how to call methods in that WebService?