Gaining band 9 can be scoring as well as tricky. Nothing comes without hard work, if you are willing to score band 9 in speaking. You should have a good command of the language. Be practical! If you have trouble in speaking English, focus on the language and not on the vocabulary. Idioms and phrases are a boon for your speaking test but only use them if you are familiar with their usage and meaning otherwise this boon might become a hinderence in your score.