Reflection is a commonly addressed design goal in commercial systems and in Human-Computer Interaction (HCI) research. Yet, it is still unclear what tools are at the disposal of designers who want to build systems that support reflection. Understanding the design space of reflection support systems and the interaction techniques that can foster reflection is necessary to enable building technologies that contribute to the users' well-being. In order to gain additional insight into how interactive artefacts foster reflection, we investigated past research prototypes and reflection-supporting smartphone applications (apps). Through a structured literature review and an analysis of app reviews, we constructed four design resources for reflection: temporal perspective, conversation, comparison and discovery. We also identified design patterns in past digital artefacts that implement the resources. Our work constitutes intermediate-level knowledge that is intended to inspire future technologies that better support reflection.