Assessing fluid status; USS and the IVC?

Fluid resuscitation represents the bedrock of initial treatment in the critically ill and injured patient with shock. In septic shock, fluid loading as part of early goal directed therapy was shown to confer a huge mortality benefit. It seems intuitive that shocked patients require fluid loading however, numerous studies in critically ill patients have demonstrated that only around 50% respond to a fluid challenge.  Are we over-resuscitating these patients and if so, what are the consequences?  Evidence is accumulating that over-resuscitation is associated with increased morbidity and mortality. Since clinical determination of fluid status in the haemodynamically unstable patient is unreliable, how do we assess volume status reliably to ensure those who need fluid get it?