Use STAR, but keep it short
Situation, Task, Action, Result gives your answer a spine. The mistake candidates make is spending 80% of the answer on the situation. Set the scene in two sentences, spend the bulk on what you personally did, and always finish with the result.
Prepare six stories that between them cover conflict, failure, leadership, pressure, persuasion and a technical win. Almost every behavioural question can be answered from that set.