The biggest mistake is to think that you can wing it without preparation. I have taught TOEFL for almost 10 years, and I have also trained other teachers to become TOEFL experts. Not even ESL teachers with no experience in TOEFL were able to get a decent score, because they did not know how to answer, what important strategies or do's & don'ts there were, or what speaking and writing format to use. Of course, once a native speaker is trained on how to take a TOEFL exam, they will do exceedingly well, and the same applies to non-native speakers taking the test. The more you prepare, the higher are your chances of a good or very good score.
In my experience, the second biggest mistake is to only practice at home in a relaxed and quiet atmosphere. TOEFL test centers are busy, and the hectic in and out of test takers is very distracting. There might be someone sitting next to you doing the speaking part while you are trying to concentrate on your writing. You may not see the other test takers and you are wearing head phones, but you can still hear others speak, and you will notice people going in and out or even walking behind you. If you are generally nervous about tests, this is only going to intensify your test anxiety. Therefore, I always recommend that my students take their laptop and practice in a busy and noisy public place, such as a coffee shop or mall.
Finally, there are many mistakes one can make in each of the four parts. TOEFL has a very particular format and you need to hire an experienced TOEFL tutor or educate and train yourself on how to best do each of the tasks. Knowing what to expect and being prepared is going to help you succeed.
Good luck!
