Make your reservation early. Many discount fares require that you make a reservation 7, 14 or 21 days before your trip depending on the fare. The best international fares often require a reservation 30 days in advance. Making a reservation as soon as you know your travel dates increases your chances of finding a fare you can live with. Flying on a weekday usually costs less. Flights on Tuesday, Wednesday and Thursday usually offer the lowest fares. Fares are sometimes (but not always) higher on Monday and Friday than on other weekdays. Saturday flights occasionally have discount fares, but as a rule it's more expensive to fly on a weekend than a weekday. Stay over a Saturday night. Most low fares require that you stay over at least one Saturday night before your return flight. However, some fares may only require you to stay a minimum of 3 or 4 days. Did we mention that you should make your reservation early? Airlines sell only a limited number of seats at the lowest fares. When those seats sell out, the price goes up. If you don't at first succeed, try an earlier or later flight. To get the lowest roundtrip fare, that fare must be available on both the departing and return flights you select. If the fare is sold out on either of these, the price you end up with will be much higher. Try an earlier or later flight if you can't find the fare you want; or, if possible, consider flying on another day. Looking for the best fares to the world's big cities? Enter a multiple airport city code, and AOL Travel will search for the lowest fare available at all metropolitan airports. Here are a few cities where this feature is valid: New York City (NYC); Washington, DC (WAS); Chicago (CHI); London (LON); Paris (PAR); Dallas (QDF); Houston (QHO). In addition, you may enjoy significant savings if you are willing to fly into nearby airports. For example, when traveling to Los Angeles (LAX), you may want to consider flying into the Burbank (BUR), Orange County (SNA) or Ontario (ONT) airports. Similarly, the cost of a trip to San Francisco (SFO) might be reduced by flying into San Jose (SJC) or Oakland (OAK), while travel to Miami (MIA) may be less expensive if you fly into Ft. Lauderdale (FLL).