There are a number of things you can do to have a better performance in the IELTS Academic Speaking Test.
First of all practice. This is the key to all the different modules of IELTS. Below you will find a separate section devoted to practising the speaking.
When you are in the test, smile and look the examiner in the eye. Try and be friendly and look as though you are enjoying the conversation. This has a big effect on the examiner. If an examiner has to talk with someone who doesn't talk much, doesn't smile at all and who clearly doesn't want to be there, then it has a negative effect on the examiner. Being nervous is fine. The examiner understands that and will try and put you at ease. But be friendly. It makes a difference.
Don't worry about the occasional mistake. The examiner will expect some mistakes - after all, English is a foreign language for you and people make mistakes in speaking foreign languages. The examiner is not making a note of every single mistake that you make. This would be impossible to do and concentrate on your speaking. He will more get a general impression of your English accuracy so individual errors don't matter. More important is your communication.
You have to talk. Without your talking input, the examiner can't grade you very well. Try and give as full an answer as you can so that you show the examiner that you are comfortable at talking at length and can communicate well. Don't do this to extremes though. When you have finished what you have to say stop. Don't try and force more out as it will probably be strained and repetitive. The examiner will see that you have finished and will give you the next question. Similarly, you won't be able to answer all questions at length. Different people can talk about different questions more and the examiner knows this. If you don't know much about something, say so and then say what you do know. When you're finished, the examiner will give you another question. You can't duck out of every question though - the responsibility is on you to talk.
Don't try and be too clever. Just try and talk normally as that is when you will perform at your best. If you try and extend yourself too much, then that is when you will make the most errors.
Perfection is not needed. You can still make some errors and get a 9 (not many errors though). So don't let making errors upset you. Get on with the talking and concentrate on your communication.
One thing that puts candidates off is that the Speaking test is recorded. This is done so that, if necessary, the speaking can be re-marked. If the bands for a candidate's writing and speaking for instance are very different, then the candidate's test is re-checked. If the speaking was not recorded, then this could not be done. This doesn't happen very often. Sample recordings are also sent to the IELTS administration to be monitored to make sure that examiners are doing a good job and assigning the correct bands. So, try and forget that the recorder is there and get on with answering the questions.
Don't forget your ID! You need it at the start of the test.
Don't give yes/no answers unless you continue with a because. It gives a bad impression. If you do give a yes/now answer, you'll probably get a why next anyway.