When you are taking the exam, be calm and relaxed. It's not going to be the end of the world if you don't pass it on the first try. (But, do study hard because the exam is not cheap with the 2009 cost at $170.) The main tool you will have on your side is your knowledge because, without that, you won't achieve success.
Remember: when you are writing, be clear with your ideas and write with coherence because a well-written paragraph can help you to achieve a higher score. Remember to study how an essay is written in English just in case you don't remember the rules of writing a paragraph.
Read every question carefully as well every reading section. It may help to write down the main idea, and then go into the sub contexts to avoid further confusion if the paragraphs or reading sections are too long and you think you might forget something.
Lastly, when you are writing the essay or other answers, be careful to write coherent sentences, and do not forget correct punctuation, English style. Hopefully, these few simple tips will help to improve your performance and score in the TOEFL.