Here is the advice from an ex-examiner.
Use the breaks to read ahead. When they give you half a minute to check your answers, don't! Use the time to get ready for the next section.
Get to the end! Miss the questions you find difficult, and make sure you do all of the easier ones. Return to the harder questions if you have time.
Writing Academic Task 1
Write a good overview. Examiners want to see a summary of the information, as well as detailed description.
Writing General Training Task 1
The tone of your letter is its character or attitude, either formal or informal. (see page5 of my lecture notes for GT part)
Writing Task 2
Spend more time planning. A good plan will help you to write a more organised essay with better ideas.
Focus on vocabulary instead of grammar or linking. In parts 2 and 3, explain your ideas in detail to increase your vocabulary score.