This was one of my biggest fears before my grade 8

as it happened for my grade 6...
What I did, as has already been suggested, is lots and lots of sightreading. Not only does this improve your sightreading, but if you play pieces that you like, you'll have fun and will be more likely to work in a productive way for other pieces too!
If there are any other areas of the exam, i.e. scales or aural that you don't feel confident with, try concentrating on them as well, as you might as well get them up to scratch if your pieces are already ok..
Hope that helped..
Good luck! *sprays good luck mist*