Well, firstly, with practice comes perfection. Take even 10 minutes out a day to speak with a fellow classmate in Spanish, and it will make a big difference! I am also available for Spanish speaking practice if you need help.

I'd also suggest watching and reading Spanish news on websites such as http://www.bbc.com/mundo. 

An important element of the Oral Exam is coming up with ideas, and at A-Level there is a lot of spontaineity. Spend time speaking with your family about anything and everything, as it will improve your ability to come up with something on the spot! You could even make a list of questions in English, as a family member or friend to ask them, and then answer in Spanish. They won't know what you're saying, but it'll help you gain confidence and hgihlight anyweaknesses in subject knowledge or grammar issues.

Remember: Content is worth more marks than grammar and fluency.