Writing
The most important thing to remember in the writing test is your audience. On the tasks it often says thing like "write a report for a university lecturer" but in fact it should say "write a report for an examiner who will look carefully for mistakes & problems". Most people think too much about the "whole" task and don't pay enough attention to the actual sentences they write or the words they use. Your answer is basically comprised of sentences so you should concentrate on producing a series of "good quality sentences" instead of "an answer". It is a good idea to work on key structures such as "conditionals" or subordinate clauses and get into the habit of producing these in your writing.
Speaking
Again with speaking it is important to remember that the examiner is listening to your language and looking specifically for problems. The most common problems that will result in losing marks are:
Too much hesitation or repetition: You may not realize that this is a problem until you hear yourself speaking in English, try to record yourself speaking English and then listen to the amount of hesitation /repetition in your speech. You will be surprised!
A lack of good vocabulary : Many people use too many "simple words" such as "beautiful, like ,dislike , big" and not enough "less-common" words or idiomatic phrases. A good area to work on is using lots of phrasal verbs as these sound quite "native speaker" and are easy to learn & use. Slang is also easy to learn and can help your vocabulary sound more "native speaker".
Grammar is an important part of the English language so you must try to show your examiner that you have some understanding of the range of English grammar. This could include using a range of different tenses, various sentence structures (simple, compound, complex). A good area to focus on is using a range of tenses ?C not just past / present / future but things like "perfect , continuous and modal forms".
Don't worry too much about pronunciation ?C the most important thing is that the examiner can understand the wonderful language that you are using. It takes years to master native speaker pronunciation in any language so your examiner is not expecting you to use "American or British pronunciation", a little work on areas like sentence stress or intonation can help to improve your score.