One thing I noticed as an examiner is that many students forget to use simple words like "and" or "but". At the same time, they overuse words like "moreover" and "furthermore".
Native speakers use "and" all the time (in both speech and writing). You can use "and" or "but" as many times as you want; the examiner will not notice!
On the other hand, if you use "moreover" or "furthermore" more than once, the examiner will notice straight away. These words do not get you a high score. In fact, in the speaking exam, they make your English sound 'forced'.