Hi Phi and others, I’ll give you some of my personal opinion, hopefully others will fix / add something for ya, too 🙂
I believe you need more practice for that part. I would suggest the following: Collect as much as possible all topics you may encounter in the speaking part, with some possible questions. Then for each set of question:
1. Try to do it as you have to do while taking the test: Think for 20mins then speak for 45 mins. Remember to check the time. Then write down what you were just spoken (this is to compare with your later work)
2. Now take into account that even though this is speaking, not writing, you still need an opening sentence and a closing sentence. You need to introduce the listeners to what you are going to say, and after all give them a short summary. Just as writing, you gotta have some main idea and some supportive details. Remake your answer with 20 mins of thinking and 45 mins of speaking. Take note for later use.
3. Now build up your speaking skill for that topics. Read related materials, memorizing vocabulary and interesting sentences that people used. For this you may use books/magazines and online resources. You may Google the topic, find it in Wikipedia, or read the Op/Ed (Opinion/Editor) from reputation magazines such as The New York Times, Times, Washington Post, USA today, Reader’s Digest, and from much much more online resources, you name it.
4. Now try to answer again: think for 20 mins then answer for 45 mins, using your new knowledge (you will see the difference 😉 ). Remember to write your down your answer again for comparison.
5. Now you can compare all 3 answers, you will see the difference. Don’t forget to check grammar errors. It will be more fun if you exchange your topics and answers with your friends.
You may find more tips/ideas/topics for this from the TOEFL Speaking forum from TestMagic. This is a huge resources with both useful and useless posts, you will have to navigate quite a bit for good information.
Good luck with your study 🙂