Hay!
The speaking exams were always the worst for me, too. They're the ultimate test of your language abilities and there's a lot of pressure to do well. What is the set-up of your exam? I was on Edexcel so it might be a little bit different.
My advice would be to make the conversation sound natural and un-rehearsed. I used to have mini conversations with myself about everyday routines and issues just to improve my fluency
If you make mistakes, don't worry too much and just keep going - one missed genitive isn't going to set you back from reaching the highest grade. The AS one is the hardest, too, in my opinion, because it's such a massive leap from GCSE and it's the first exam where you actually have to think on your feet! The examiners are not expecting perfection.
I made some horrific mistakes in my AS one, such as calling Victoria Beckham a man and saying that she was 'zu glatt', because I forgot the word for skinny and could only think of straight! My examiner was honestly trying so hard not to laugh.
I also screwed up right at the start by asking to start again in English... haha
Don't have the mindset that you'll do badly; try and be as confident as possible (I know this is easier said than done!)