Buy upgrades on low-fare airlines such as America West

Airfare 101 is a course that’s always changing. Let’s review the past year’s lessons (and some old stand-bys) and get ready for the changes 2005 is bound to bring:

Buy inexpensive first- and business-class seats on low-fare airlines including AirTran, America West and Spirit. These seats are a fraction the price of front-cabin service on legacy carriers. Spirit, AirTran and ATA even offer day-of-travel upgrades ranging from $25 to $75.

When booking travel for two, choose an aisle and window seat, leaving the middle seat open. Chances are it won’t be assigned unless the flight is full.

Most lower-fare round-trip tickets come with a 30-day maximum-stay requirement. If you need a longer stay, look for tickets sold on a one-way basis (usually on low-fare carriers) or buy from a carrier such as Alaska Airlines, which removed maximum-stay restrictions.

Travelers who are students, teachers or younger than 26 can purchase low-cost international fares at www.statravel.com and www.travelcuts.com. Some deals can be used by travelers of any age. To purchase some fares through STA or Travel Cuts, you’ll need a $22 student-travel card (also useful for other discounts). Contact: www.isiccard.com.

For domestic travel for children, consider Southwest Airlines’ Infant, Child and Youth fares. You can often do better using sale prices, but these fares are fully refundable and have no advance-purchase requirement. Infant fares (for children younger than 2 occupying their own seat), range from $48 to $129 one way. Child and Youth fares (ages 2 to 21) range from $75 to $284 one way.

Don’t waste money on avoidable fees for extra pieces of luggage or overweight baggage. Most airlines allow two pieces of checked luggage per passenger at no more than 50 pounds each. Each added bag incurs a fee. Fees for overweight bags start at $25. AirTran, JetBlue, Southwest and Spirit allow a 70-pound per bag limit and let each passenger check three bags.

Alternate airports not only tend to offer lower fares, but they also are usually less crowded. Locate alternate airports at www.alternateairports.com.

Many tour operators use charter flights. Some sell unused seats (airfare only) at deeply discounted rates. The closer to departure date, the lower the fares. Good sources include WorryFree Vacations (888-225-5658; www.worryfreevacations.com) from nine U.S. cities (including Dallas) to Las Vegas, Hawaii, the Caribbean, Mexico and Costa Rica.

You could dramatically cut the price of international travel in business class by purchasing through a code-share airline, rather than its U.S. counterpart. This can hold true even when the U.S. airline is selling seats on the international carrier’s flight. You’re most likely to find these deals on trans-Pacific flights (Korean Air over Delta, for example, or Air China over United), though they can also exist on European flights (Iberia over American, for example).

One of the handiest guides for travelers, “Airport Transit Guide,” covers the best and least expensive ways to get to and from 456 airports worldwide. It can be ordered at http://airporttransitguide.com for $9.95.