The UML (Unified Modeling Language) has been widely accepted as a standard language for object-oriented analysis and design. Among the UML diagrams, one of the most difficult and time-consuming diagrams to develop is the object interaction diagram (OID), which is rendered as either a sequence diagram or a collaboration diagram. Our experience shows that developers have significant trouble in understanding and developing OIDs. In this paper, we present an effective heuristic for developing interaction diagrams and illustrate the technique with a case study. We found that students effectively developed OIDs using this heuristic method.