Cut out all chocolates, crisps, fried foods i.e. junk food for at least 1 month b4 your event.
Drink LOTS and LOTS of water (but don't over do it bcos some ppl actually can die of over-diluting the salts in their bodies.) I think they usually say 8 glasses a day is enough.
Eat lots of salads and fruit and fish. I heard something about trying to cut down red meat, but if u do, make sure u take iron supplements.
A colleague used to eat a grapefruit twice as a snack, (meaning she ate properly at other times) and since she started doing that, her hair and skin became really healthy looking. But thena gain she also cut out unhealthy fats etc.