If you're choosing between several flights, inquire as to how full each flight is and then select the least crowded.

If you're traveling with one other person, ask for seating assignments in the aisle and window seats in the same row on the same side of the plane. The middle seat, which is typically assigned last, may well remain empty, and you'll each be next to an empty seat. (If someone is assigned to the middle seat after all, one of you can simply ask to switch places with that passenger, who will probably be only too glad to give up the middle seat, allowing you two to sit together.)

Depending on the seat configuration of the aircraft, it can also be a good idea to request a seat assignment in the center section toward the back of the plane, since the side sections of seats near the front fill up first.

Once you arrive at your gate, ask the agent to place you next to an empty seat if one is available. If the flight is not full, the agent can often accommodate this request.

If there's a particular location within the plane that you want to occupy, let the airline know in advance. Most airlines can provide printed brochures of the cabin configurations for the aircraft they fly; obtain a brochure in advance, then request a specific seat number when making your seat assignments.

If your legs get cramped, you might volunteer to sit in one of the emergency-exit rows, which provide more leg room and often have one fewer seats than a regular row. Be aware, however, that seats in emergency-exit rows sometimes do not recline.

The emergency-exit row is a safe haven for those who prefer a kid-free flight experience, as children are not allowed to occupy these seats. Another good way to avoid small children is to avoid bulkhead rows, which are often occupied by families.

In many cases, airlines let you change the return flight of your itinerary for a fee of about $50 to $75 in addition to any increase in fare.

Changing an entire unused ticket or the outbound flight of your itinerary, however, is another story: These changes tend to be more costly. Most airlines will charge a change fee plus the difference in price between the old ticket and the new itinerary. Your best bet is to plan carefully and avoid making changes whenever possible. If this is unavoidable, reschedule your flights as soon as possible, increasing your chances of a smaller difference in fares.

If you are a traveler whose plans frequently change at a moment's notice, try to fly business or first class. Not only will you be more comfortable, you will also find that it's usually easier and often free to change business and first-class tickets.

Think about buying unrestricted tickets. While they tend to cost more up front than restricted tickets, they typically carry no charge for changes. Unrestricted tickets are also refundable, while restricted tickets can only be exchanged.

Perhaps the best advice is to fly in the morning. This makes sense for two main reasons--you won't be subject to delays that often disrupt later flights, and if an early morning flight is canceled or delayed, there are more re-scheduling options.

Whenever possible, choose a non-stop flight. But if you must make connecting flights, select routing through the least congested airports. Additionally, you should try to choose connecting cities with the least chance of inclement weather, the main cause of delays and cancellations.

Before choosing a particular flight, check its dependability rating.

Flying on a weekday usually costs less.
Flights on the off-peak days of 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 three or four days.

One airline is better than two.
It's almost always less expensive to use only one airline for a trip instead of two. Booking two airlines can, in some cases, cost hundreds of dollars more.

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.