Practise, practise, practise. Practise speaking on different topics similar to the test. Perhaps ask someone to give you a topic. Just talking to someone will also help. This will help you to 'think on your feet' - to get the words to flow, so that you don't hesitate too much. Get a native English speaker to listen to your speech and point out any errors. There is no point repeating and reinforcing incorrect pronunciation or words.