I never took the toefl ibt before, so I really don't have a real experience about the test and especially the speaking section. But, from my friend experience and how hard it was to think in 15 seconds and gave a complete answer in 45 seconds, she told me that I have to practice a lot on several speaking prompts to be able to answer smoothly in the exam.
So here is my advice:
1- Try to get speaking prompts as much as you can for the two independent speaking tasks and try to get ideas about each one so when you are in the real exam, you don't have to think about any thing new. all you have to do is to remember what you have already practiced.
2- Try to record your answer for each prompt and save it to your i pod ( if you have one) and hear it over and over again till you memorize all the answers.
3- Time yourself when you record the answers, so your answer has to be between 43-45 seconds.
I hope my advice can help you.