Get a pair of round glasses without lenses and put a piece of clear tape around the middle of the frames so that they look just like Harry's broken glasses.
Wear a long-sleeved rugby shirt, orange with a red stripe across the chest like Harry and jeans and sneakers.
Apply mousse or gel through your hair to make it stand on end. Use Halloween makeup to create Harry's trademark pink scar in the shape of a lightning bolt in the middle of his forehead.
Tie a red cape around your neck if you want. You can carry a broom stick too.
Thanks to Jay for this neat wizard idea!
Harry Potter: Wear cape over a collared shirt and black pants. Carry a wand or small broomstick in your hand. Buy cheap black plastic sunglasses and pop the lenses out. Wear a pointed hat.
Thanks to GoofyGirl for this idea!