In many areas, you can save money on cooling bills by opening up your windows in the mornings and evenings, when the temperatures are typically more pleasant.
Cross ventilation works by taking advantage of high and low pressure zones created by wind to draw fresh air through a building. Breezes enter through a window or vent, bringing fresh air, while the pressure difference on both sides of the building pulls stale air out an opening in the other side. The key, of course, is to have open windows (or vents) on opposite sides of the structure.
Get the best results by working with the predominant direction of winds. You can also set up fans to alternately push or pull air from outside in.
Cross ventilation is also great for ventilating indoor air, which can be more polluted than outdoor air.