Image: Airplane at gate © Brand X, Jupiterimages

Once upon a time, airlines advertised their full prices, making it easy to choose among different carriers.

Today, airlines advertise low prices upfront and then stick you with fees or back-end charges you might not have expected. Predicting the cost of an airline ticket requires a calculator and a magnifying glass to read all the fine print. Even then, you may be in for some unpleasant surprises.

Airlines raked in $3.4 billion in baggage fees alone last year, a revenue source that's largely responsible for turning the industry's red ink to black. More fees are coming -- Spirit Airlines just announced a $5 fee to print a boarding pass, for example -- as carriers continue to "unbundle" the flying experience and figure out more ways to profit from services that used to be free.

"Airlines are squeezing every last dollar from every last customer on every last flight," said Tim Winship, the publisher of FrequentFlier.com.

Here are some pesky airline fees to watch out for, and how you can get around them:

Baggage fees

Travelers know by now that most major airlines charge to check luggage, and the typical fee is around $25 each way for the first bag and $35 for the second. (The exceptions are Southwest, which allows two free checked bags, and JetBlue, which allows one.)

What travel customers may not know is that a bag that is a single pound over the 50-pound weight limit can trigger up to $100 in additional fees -- one way. An oversize item can cost a breathtaking $300 to check one way.

The airlines have "all pretty much gotten in line on the $25 fee" for the first checked bag, Winship said. "The other fees don't get as much attention, so the airlines feel comfortable going to town on them, knowing they're not getting called on it."

And then there's Spirit Airlines, the only carrier (so far) that charges for carry-ons as well as checked bags. Spirit's fees for carry-ons range from $20 for members of its fare club to $45 at the gate, versus $18 to $38 for the first piece of checked luggage.

How to cope: Current reservation systems don't allow you to add in the varying costs of checking baggage so you can get an apples-to-apples comparison on how much flights actually cost. Instead, you can download an updated guide to airline fees at SmarterTravel. Refer to it before booking your travel.

Liz Weston

Liz Weston

Award ticket fees

You joined your airline frequent-flier program to earn free flights, right? Good luck with that.

Airlines are starting to lard fees onto award tickets. You'll often face a $25 to $35 "booking fee" if you need to talk to a representative on the phone. And that's something you often must do because navigating the airlines' award redemption system can be so complicated and because calling a rep is sometimes the only way to get the flight you want, Winship said.

Changing or canceling an award ticket can trigger fees up to $150, while some airlines impose a $50 to $100 fee for a "last-minute" award booking -- generally any reservation made within three weeks of the flight.

Many airlines now also charge a cash "co-pay" if you want to use award miles to upgrade a coach ticket to first class. American Airlines, for example, charges $50 plus miles each way for a domestic upgrade and up to $350 each way for an international upgrade.

"That can cost you $700 for a round trip," Winship said. "You're talking about significant money."

The fees vary based on the cost of the ticket, with cheaper tickets triggering higher fees.

Another fee to watch for: US Airways charges $25 to $50 simply to issue an award ticket. It's so far the only domestic carrier to do that, and Winship finds the fee "an outrage."

How to cope: If you have to use a phone rep to book an award flight, ask for the fee to be waived. The rep may not cooperate, but it's worth asking. The other fees are harder to circumvent. You can avoid the cash co-pay for upgrades by redeeming miles for a first-class ticket, but that assumes you have enough miles and want to deploy them this way.

Fuel surcharges

Charging extra for fuel hasn't really caught on yet among U.S. airlines, which instead have been raising fares or imposing "peak travel" surcharges. But foreign airlines have embraced these sometimes-spectacular fees. Where you'll really get stung is in trying to book trips on these airlines using frequent-flier miles.

British Airways, for example, recently levied a $346 fuel surcharge on economy-class roundtrip tickets from Los Angeles to London; a first-class seat would trigger a $592 surcharge. These fees are embedded in the tickets' cost when you pay cash, so the $858 British Airways charges for the flight, including the surcharge, is about the same as the $889 Delta charges for the same route. But you still have to pay the surcharge, even when you're using miles to schedule an award flight.

With all the surcharges, taxes and booking fees added in, the economy flier would pay $567 in fees, taxes and surcharges for the British Airways flight, while first-class fliers would pay more than $1,400 for an award ticket -- in addition to the miles they cashed in.

These costs "seriously undermine the value of the frequent-flier miles that you've earned," Winship said. "The whole value proposition is upended when you have to factor in all this cash you have to pay."

How to cope: Consider using your miles on a foreign carrier to book a seat with one of the airline's U.S. partners that charges far less cash to book award flights. You can use British Airways miles to book American Airlines tickets to South America or Cathay Pacific Airways tickets to Hong Kong, for example. The other foreign airlines that charge significant fuel surcharges -- Air Canada, Lufthansa, Virgin Atlantic and All Nippon Airways -- also have U.S. partners that don't.

Seat selection fees

Most U.S. airlines charge extra if you want a good seat, and more will follow, Winship predicted. About a third charge if you want any say in selecting your seat, while several levy fees for "premium" seats with more space.

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"In the past, it was first come, first serve, so if you were quick enough and savvy enough to book an exit row seat, it was yours for no extra fee," Winship explained. "Now airlines are seeing (seat selection) as yet another source of extra revenue."

What's holding the other airlines back isn't a lack of will to charge, Winship said, but older reservation systems that don't allow them to charge customers for selecting seats. Eventual upgrades will change that.

How to cope: Again, download SmarterTravel's handy fee-comparison sheet, and refer to it before you book a flight. It may be worth paying extra: Sometimes premium seats come with priority boarding, which can be handy if you want to find precious overhead bin space for your carry-on, and United's package of perks includes access to a shorter security line at many airports.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.