Note: this answer is valid for improving in any language at any level.
Key words: practice and revision.
- Practice will make your words flow faster and as a result will make you feel more confident. Regular revision will make memorising vocabulary and new grammar rules easier to remember and, in time, easier to insert into speech.
- Your examiner will give you marks for the originality and clarity of thought as well as for your accuracy, so make sure you have clear opinions to express when you answer questions.
- Practice understanding and pronunciation using audiobooks, films and documentaries: the inflections and accent will affect the way you speak more than you think.
- Find someone you can practice with. It can be embarrassing and awkward, but if the help is mutual and serious, a lot can be achieved that way.
- Find topics you are actually interested in (films, contemporary dance, medical ethics, political elections, science VS religion, etc) and practice forming clear, well-articulated opinions on them. The difference your interest in a topic can make is phenomenal. And the more complex it is, the easier it will be to express simpler opinions on more mainstream topics in the exam.
- Sometimes, very informal settings help with the flow of conversation. Practice with a friend/relative around dinner, while you're walking to town, as you're doing your grocery shopping, etc... Spontaneity works best!
- Also, and finally, grammar. Whilst perfect grammar is not vital to making yourself understood, having good grammar skills is essential to getting a good grade, and if you start off right then you won't slip into bad verbal habits and have to 'unlearn' them at a later date. There are plenty of ways to improve with the modern blessing of technology: websites that make a game out of it; tablet applications; daily brain workouts; etc.
Above all, keep at it. Repetition is key.