Long-term behaviour change is essential to many societal and personal challenges, ranging from maintaining sustainable lifestyles to adherence to medical treatment. However, prior research has generally focused on interventions dealing with bounded, present-tense, and discretely measurable behaviour change problems, evaluated via relatively short-term trials. This has led to a skewed prioritisation of behaviour change techniques and left a critical gap in design guidance. Hence, there is an urgent need to (i) examine how behaviour change techniques can be abstractly prioritised and (ii) related to contextual, embodied interventions during long-term behavioural design. We address this need using a Delphi survey method with 12 international experts on behavioural intervention complemented by a reanalysis of over 100 real-world cases. This provides the basis for examining how experts prioritise the Behaviour Change Technique Taxonomy (BCTT) for the long-term, as well as how this corresponds to real-world long-term interventions. Based on this we provide essential, and as a first, guidance for long-term behavioural design as well as contributing to wider research on how to deal with the demands of long-term behaviour change.