Every time you practise, pick out a short piece that you haven't played or looked at before. Look at it for around 30 seconds (that's the time you'll get in the exam) and then play it straight through. Don't stop or go back to correct mistakes, just keep going.
Things to take note of during the 30 second observation time are; tempo/style, time signature, key signature, and any awkward looking bars or accidentals. If you like, you can try playing some of the bars during that observation period.
It will probably help if someone else does the 30 second clock watch for you.
Try to make your attempt as musical as possible, if you make a slip don't worry, accuracy is obviously important, but you will also score marks for musical shape too, so whatever you do JUST KEEP GOING.
The more sight reading you do the better you will get. It's as simple as that.
Good luck