First of all, make sure you are actually ready to take the exam. Get your teacher to be honest with you, and if the answer is "no", don’t put yourself through it.
Secondly, make sure you are choosing the right pieces for you from the available list. Even though they are supposedly all of the same standard for the grade, some ARE easier than others. They are usually the ones that are very popular, and that every other student playing the same instrument at the same grade as you is also working on towards their exam. Don’t make life difficult, do the technically easier pieces! There are no extra marks for doing things that are really technically beyond you.
Thirdly, scales, scales, and more scales. You can pick up a lot of points on scales, and exams can be passed and failed on scales. There’s no reason why even the most hit and miss student shouldn’t pick up FULL MARKS on scales. An examiner is looking for accuracy of notes, and not necessarily a beautiful sound, consistent dynamics, or musical phrasing. I’m not saying you shouldn’t work towards these ends, but here, the notes are all.
Fourth, sight reading. There’s no magical gift to sight reading. There are simple tricks and tips for making it easier for yourself. Take your time to look at the time and key signatures. Look ahead for accidentals, and for any difficult passages. Think it through in your head first – you don’t need to be able to "hear" the tune, just think through the rhythm.
Fifth, aural tests. There’s no easy answer to the aural tests, as they do depend on a certain amount of aptitude. Find someone who plays the piano to practice these with you beforehand. I’ve been a music teacher for over 20 years, and I’m still staggered by the number of students who think this is an area that doesn’t need work! Get a sample book, and look through them. I’m presuming you’ve done previous grades? Where did you make mistakes then? Can you improve on that? Don’t be afraid to ask the examiner to explain anything you don’t understand.
Finally, some hints on preparing for and actually taking the exam:
Practice a little EVERY day in the month leading up to the exam.
There is no need to memorise your pieces, but if you could do them without the music, you will be much more confident.
Go through every part of the exam in your mind beforehand. Think about what you will wear – you don’t want to be put off by uncomfortable shoes, or noisy jewellery, but it’s surprising how often this can happen! Imagine walking into the room, greeting the examiner, and successfully completing the whole exam.
Get up and get ready in plenty of time on exam day. Plan a treat for after the exam.
Greet the examiner and smile warmly. You will be a bit nervous, and they know you will be a bit nervous, but whatever you may think, they want you to do well. Examiners are very, very experienced music teachers who have taught for many years. Many years ago, however strange it may seem, they were standing exactly where you are right now.
Best of luck – I’m sure you’ll be fine! You might even get me…