This report is the outcome of a joint activity between Science Europe and the OECD, and presents a generic framework for improving the use and operation of national research infrastructures (RIs), which play a key role in enabling and developing research in all scientific domains and represent an increasingly large share of research investment. It includes two guiding models: one for portfolio management and one for user-base optimisation. These identify the key principles of an effective national RI portfolio management system and the factors that RI managers should consider with regards to optimising the user base of national RIs. Both guiding models account for the diversity of national systems and RI operation approaches. The report also contains a series of more generic policy recommendations and suggested actions for RI portfolio managers and RI managers.