My suggestion is:
1. Take about 15-30 seconds to think about the answer, then answer the question. (Like the speaking test in TOEFL iBT).
2. Answer it right away by rehearse the question in complete sentence while you're thinking about the answer. This way is the one I always take.
For example,
Q: What do you think about .... bla bla bla?
A: (say this in slow tempo, like Andante or Adagio, while u're thinking about ur answer) What I think about ...... bla bla bla is (start telling your answer at this point).