Despite the availability of computer-based alternatives both for desktop and touch screen systems, a number of cooperative work processes still commonly rely on simple paper sticky notes. In this paper, we present the first in-depth investigation of the real-world practices of people who use paper-based affinity diagrams and similar clustering processes in their work, in order to identify challenges and requirements for technology support. Findings from retrospective and artifact-based interviews with 13 participants suggest ways in which the rich interactions and material affordances offered by paper are key to the process. Instead of seeking to replicate interactions with paper on a screen, simpler transfer of information between the physical and digital worlds has the potential to address many of the most pressing problems experienced in practice. We describe different types of technology integration and augmentation, with preliminary recommendations for different situations.