Another tip snipped from the Revit Family Editor Tutorial for Revit 2008 as mentioned in the previous post. You can download the PDF copy HERE.
Fittings (duct, pipe, conduit and cable tray fittings) expect the instance origin of the family to be the (projected) intersection of the connectors. In most cases for fittings, there is a point on the fitting where all of the connectors (if extended into the fitting) will collide. Fittings expect this collision to be placed at the original intersection of the Center (Front / Back), Center (Left / Right), and Reference Level work planes. For this reason, it is good practice to pin these reference planes before beginning to build the family.