The only way is to practice. Since I don't know what grade you are or what instrument you play, I can't say anything very specific. However, for sight reading, get the ABRSM graded sight reading books from a music shop (if you play an instrument for which they are available), but save those as the last sight reading practice you do before your exam as the length and standard are likely to be the most similar to what you will get in an exam. If you don't already have them, get the ones from grades lower than your standard, and work your way up. Practice sight reading pieces in your books that you've never looked at before, and borrow books from other people who play the same instrument so you are sight reading pieces which are truly unfamiliar.
With each piece you sight read, there are a few obvious things to look at (sorry if I'm insulting your intelligence here, but as I said I don't know your standard and your experience of sight reading, so please forgive me). Firstly, look at the key signature, as this is a common source of mistakes. Secondly, look at the time signature, and work out the speed, counting to yourself. If you're in exam they give you thrity seconds or something to look at the piece before you begin to play, and I think you're allowed to try bits out so it may be worth spending time working out e.g. the exact rhythm for bits that look difficult. The main thing is just to stay calm and keep going... It's better to play the piece correctly but slower than the meter than it is to play it at the right speed but riddled with mistakes. Don't let mistakes get you down either - sight reading is rarely perfect and I've always got good marks for mine with a couple of mistakes, even in grade eight.
For aural, get the ABRSM books and CDs (it was tapes in my day but I imagine it's CDs now). Go through these in your own time, as there are many examples so you shouldn't run out of new exercises to try. Your teacher can help you with the aural a lot more than they can with the sight reading so ask to do some practice exercises in your music lessons, but don't just leave it until close to the exam, do them all year round so you are used to them and when an exam comes up they don't seem like a big deal.