1-2 hours before your exam eat a nice balanced meal with some Low GI Carbs (rice, brown pasta, apple... something like that.) Try not to eat anything too heavy before an exam, espeesically with a Low GI, it'll make you more sleepy than alert as complex carbs tend to help you sleep and the carbs won't be available during the exam anyway!
Directly before your exam eat a bananna or orange, seriously, it has really helped me through my 1st year medical exams so far! Anything with a high glycaemic index before an exam will really help as the sugar is available straight away and hopefully will keep you a little more alert.
Oh, and keep very very hydrated, the last thing you want to do for your brain is deprive it of water!