Travelers have to be more strategic to avoid some airline fees. Before you buy tickets, read the airlines' contracts of carriage so you know what kind of fees you may encounter. Read the section on baggage if you plan to check more than one piece, and look at the prices for changing tickets.

United recently began charging $25 for a second piece of checked luggage for coach-class customers on domestic flights. The airline estimated it would make a profit of $100 million with this policy. US Airways, Delta, Continental, Northwest and Air Canada have joined United in charging this fee. On most of these carriers, if you have elite frequent-flier status, you can avoid the charge.

It's not only the legacy carriers that are instituting these fees. AirTran will begin charging for a second bag on May 15, but the charge will be $10.

These fees apply to luggage that meets normal size restrictions. If your bags are overweight or oversize, plan to pay more. On United, for example, if a bag exceeds 62 linear inches, you will be charged $100 extra per bag. If a bag weighs more than 50 pounds, you'll pay $100 per piece. If a bag is overweight and oversize, the fees will be combined. The carrier will not accept bags more than 115 linear inches and-or more than 100 pounds.

You'll also pay if you are checking bags beyond the second bag. On US Airways, your third through ninth bags are $100 each for luggage weighing 50 pounds or less, $150 each for bags weighing 51-70 pounds and $200 each for bags weighing 71-100 pounds. On Delta's domestic flights, the airline charges $25 for the second bag, $80 each for bags three, four and five, $110 each for bags six, seven and eight, and $180 each for bags nine and 10.

The $25 fee for a second piece of luggage applies to travel on or after May 5 for most airlines. The United policy applies to tickets purchased on or after Feb. 4 for travel within the U.S., Puerto Rico, U.S. Virgin Islands and Canada. US Airways' fee applies to tickets purchased on or after Feb. 26, for travel to destinations other than Europe and Asia.

On the local front, Southwest still allows two free checked bags. Coach passengers on American who buy tickets on or after May 12 will be charged $25 for checking a second bag. The charge applies to travelers within the U.S., Canada and U.S. territories, such as Puerto Rico and the U.S. Virgin Islands. Passengers who buy nondiscounted tickets or are premium members of its frequent-flier program won't be affected.

These luggage fees are not the only increases. United recently announced that it raised its ticket-change fee on nonrefundable fares from $100 to $150 on North American flights. The change took effect on April 19, so if you purchased tickets before that, the old change fees should apply.

Travelers should print a receipt showing the date of their ticket purchase and print the contract of carriage at the time of the purchase. If new fees are announced after you purchase your ticket, you'll have a reference to back you up to avoid the fees.

Tom Parsons is publisher of Bestfares.com.