The speaking test is an opportunity for you to really show off your English.
Keep the conversation flowing and hold up your own speaking part, and remain on topic.
Avoid too many pauses and elongated pauses. You might naturally pause because you're speaking, but you don't want longer pauses.
Avoid repeating yourself and your vocabulary. You want to demonstrate your vocabulary and show your range.
How can I prepare for the speaking test?
When practicing, try to think of the different avenues the topic could take. A useful way to practice this with all of the topics, is to create a mind-map.
Get down everything you can possibly say about the topic in 15 minutes and you will surprise yourself with how much there is to talk about.
Try using this image to generate questions about this topic, and have a go at mind-mapping yourself