Why impromptu answers ramble
Most rambling answers do not fail because the speaker has no ideas. They fail because every idea arrives with equal importance. PREP gives the answer a simple order.
Point, Reason, Example, Point
Start with your answer, explain why, prove it with one example and then repeat the main point as a conclusion. This works for opinions, interviews, meetings and classroom answers.
Practise it under time
Use a 60-second timer. Spend ten seconds on the point, twenty seconds on the reason, twenty seconds on the example and ten seconds on the close.