Running air conditioning can be very expensive, but you still need to cool down at night so you can sleep comfortably. Real Simple notes that a fan blowing air out of your room is better than one blowing in.
Photo by fauxto_digit
When your fan is facing out of the window, it blows hot air out of the room, which his replaced by cold air from outside. As the cold air comes in, the temperature will drop. It's best if you have another window to open elsewhere so you can get a cross draft going too.
Keep in mind this only works at times when the outside air is actually cooler than the air inside -- if that's not the case, you'll want the fan blowing directly at you.