I know the feeling, being a bit of a waffler myself.
Having a brief tactical pause could help, as mentioned - I will have a drink there to swig on, to give me a moment to reflect on the question before jumping straight in with a rushed answer.
The 69 questions from the application are a useful guide as to what NTS questions may be asked (in external/non signaller interviews). Review these, and consider how you would answer if you were asked them, or similar, in the interview itself. This should allow you to practice, and also potentially improve that part of the application for future jobs. Ideally you will then have some solid examples ready to go, already structured, that can be used for each NTS.
More interview practice - friends/family can help - will often make the real thing flow much better.