How can I improve my control over the flow of a speaking assessment?

It is important in examinations to cover a wide variety of subjects in order to best demonstrate the breadth of your understanding both of topical issues and the language that can specifically relate to them. In an oral exam, while it is of course paramount that you directly address the question that you are asked, you should try to remain calm and have an open mind so as to add additional comments at the end of your response to lead discussion to another topic. A great way to become more accustomed to doing so would be to create a concept map of all of the topics that you have covered over the year and then create visual links between subjects where there are similarities.
For instance, when discussing issues surrounding health and fitness a link could be made to environmental issues by discussing the advantages of the Vélib initiative in that it is beneficial to use bikes for both your cardiovascular health but more importantly because they are better for the environment as they do not produce carbon emissions.
By introducing a new, related issue at the end of your response, it will be natural for the examiner to then enquire into the new topic without it feeling uncomfortable or forced. Additionally, by using a concept map and familiarising yourself with the relevant links and how subjects relate to one another, you are much better equipped to answer a wider range of questions than if you memorise specific answers. This greatly improves your spontaneity during the discussion.