Unless your travel dates are absolutely set in stone, use flexible date searches for flights. Just about every airline's website offers the feature — in which you can retrieve prices for a few days before and after your selected dates — as do major sites like Travelocity and Kayak. It tends to be cheaper to fly on days other than Friday and Sunday, but don't assume anything. Flying just one day earlier or later than your ideal date could mean saving an easy $100.