Keep it simple!
This tip refers to the whole Speaking test. Don’t start long and complicated sentences if you don’t know how to finish them! Keep your sentences simple, and words - understandable. If you make grammatical error – it’s ok to correct yourself, but don’t overdo it, you must sound fluent.
When you hear a question, your mind automatically draws an image with the answer. You can describe this image in your own language, but when you start to describe it in English, suddenly you don’t have enough words in your vocabulary. So try to think of what part of that image you can describe, do it and stop there. Don’t let yourself show what words you don’t know by saying incomplete sentences.