Agile methods and product line engineering (PLE) have both proven successful in increasing customer satisfaction and decreasing time to market under certain conditions. Key characteristics of agile methods are lean and highly iterative development with a strong emphasis on stakeholder involvement. PLE leverages reuse through systematic approaches such as variability modeling or product derivation. Integrating agile approaches with product line engineering is an interesting proposition which – not surprisingly – entails several challenges: Product lines (PL) rely on complex plans and models to ensure their long-term evolution while agile methods emphasize simplicity and short-term value-creation for customers. When incorporating agility in product line engineering, it is thus essential to define carefully how agile principles can support particular PLE processes. For instance, the processes of defining and setting up a product line (domain engineering) and deriving products (application engineering) differ significantly in practices and focus with implications on the suitability of agile principles. This paper presents practical experiences of adopting agile principles in product line planning (a domain engineering activity). ThinkLets, i.e., collaborative practices from the area of collaboration engineering, are the building blocks of the presented approach as they codify agile principles such as stakeholder involvement, rapid feedback, or value-based prioritization. We discuss how our approach balances agility and the intrinsic needs of product line planning. A case study carried out with an industrial partner indicates that the approach is practicable, usable, and useful.