Making passive measurements is a challenging process. A small number of research groups have overcome these difficulties and have made terabytes of captured network traffic available to the Internet community. We have discovered that even when initial hurdles to collecting traces have been overcome, there are potentially many methodological weaknesses that can severely limit the usefulness of the data collected. This article describes these weaknesses and suggests methodologies to avoid them. Five case studies from the work of the University of Waikato's WAND network research group are presented. These studies illustrate the need for care at all stages of the measurement process, analysis, and presenting the results.