Native speakers run the risk of getting a lower score than they should through over confidence. Just because it's your first language doesn't mean you can sit back and get 9s straight across. The S/W examiner doesn't care if you are from Bristol, Sydney, or Timbuktu...they just go by what you present to them.
I am a native speaker and the first time I took the test I only scored a 7 on the reading section. After having honed my scanning/skimming skills, as well as adjusting my strategy (first quickly skimming text, underlining relevant bits, then going back to those bits and skimming/scanning to get the answers t), I got a 9 the second time.
Like with any other standardized test, the test-taker needs to become fully familiarized with the format of the test, and how grades are allocated.
The Cambridge practice tests volumes 2-8 are actual tests that have been retired, and you should use them to practice a couple of times before the real thing. Each volume has 4 complete tests. Make sure you study the model answers in the back and go through your errors in the L/R very, very thoroughly to understand why you made them.
For the long turn in the speaking section, try to speak for the whole two minutes and try to be as communicative and natural as you can.
If you must speak on a topic that you don't care for in the long turn, that's tough, but you don't have a choice. If that happens (it shouldn't because the questions are supposed to be answerable by just about anyone), just go with it and either fabricate something (stay on topic, though!), or talk about why it's a bad topic for you. For example, I would be at a complete loss if I were asked to talk about a sport that I played as a child, so in that case, I would just talk for two minutes about how I didn't like sports in my formative years, and perhaps some experience that happened to me when I reluctantly had to play a sport. Remember, the examiner is assessing your communication ability, not your knowledge on a topic, and it's only 120 seconds that you have to talk for.