WITH FEES FOR

everything from extra baggage to a bag of in-flight chips, it's next to impossible to fly somewhere these days without being dinged by added charges.

Blame the airlines' nickel-and-diming ways on a month's worth of oil prices that have topped $100 a barrel. By the International Airline Transport Association's estimates, every dollar jump in oil prices generates an additional $1 billion in fuel costs for the airline industry. "Right now, the airlines haven't met a revenue source they didn't want, need or like," says Terry Trippler, owner of TripplerTravel.com.

At first, carriers tried to improve their profit margins by cutting staff and increasing fares. But in order to stay competitive with low-cost carriers, they could only boost ticket prices by so much. (Because the Department of Transportation requires airlines to tack on fuel surcharges in the base price of the fare, directly translating their added costs to your ticket would likely price them well beyond competitors.) And so, the airlines are seeking other sources of revenue that can be charged well after you've booked your ticket.

Delta, for example, just increased fees for everything from curbside check-in (now $3 a bag; it was free) and oversized bags ($150, up from $100) to phone booking ($25, up from $20) and traveling with pets in the cabin of the plane ($100, up from $75). On top of that, a new $25 fee for checking a second bag will go into effect May 1.

Delta's fees are just the latest in a litany of new charges announced by the major carriers in recent months. As is the case here, once one carrier starts charging for a given convenience, the rest quickly follow suit. "It's never long before the others realize, 'gee, that wheel rolls, we don't need to reinvent it,'" says Stephanie Abrams, host of the Business Talk Radio show "Travel With Stephanie Abrams."

Unfortunately for travelers, if they want to avoid being charged for services they'll likely use, they'll have to do some extra research before buying their ticket. Instead of booking with an airline that offers the lowest fare, it's important to factor in the added cost for such things as the number of bags you plan on checking and whether you'll want food or extra legroom during the flight. (You'll need to hunt down policies on individual airlines' sites to accomplish this.)

Want an easier solution? Keep your travel simple. "The consumer who will fare best is one who travels light" literally and figuratively, says Anne Banas, executive editor for SmarterTravel.com, a travel advice site. The more you require services or make any special requests, the more likely fees will be attached.

Use our guide to see four increasingly common charges, plus which airlines are still (somewhat) fee free:

Overpackers

"They're watching checked bags like hawks," warns Trippler. After all, the number and weight of bags in the cargo hold has a direct impact on the amount of fuel the aircraft uses. Come May, Delta, Northwest, United and US Airways will all charge $25 to check a second bag. American has filed paperwork in Canada which requires advance notice of fee changes indicating it is considering adding such a fee. Last year, Spirit began charging $10 and up for any checked items.

These added fees have spurred more people to lug carry-ons onboard. So even if you're traveling light, be prepared for less space in the overhead bins, and stricter rules on what can be brought aboard, warns Trippler. Delta has already rolled out a carry-on approval tag that must be on all bags.

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A Decent Meal

American Airlines' regional carrier American Eagle pioneered the $1 soda in 2006, and then swiftly backed off when consumers complained. "But oil wasn't above $100 a barrel when they tried it," says Trippler. "The dollar soda...could happen any day." Airlines continue to finesse pay-for food menus, offering everything from $5 snack packs of cheese and fruit (United) to $2 potato chips (Northwest). American Airlines is currently testing new menus that include Glac au smartwater ($3) and a combo meal consisting of a Boar's Head turkey sandwich and chips ($10).

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Kids Traveling Alone

Some 20,000 minors will travel the airways solo this summer to visit relatives or attend camp, estimates Tom Parsons, CEO of

BestFares.com

. But with increasing air traffic and

flight delays

, airlines are asking parents to fork over more cash to ensure children make a safe and timely journey from Point A to Point B. For tickets issued after April 1, Northwest and Delta are asking $100 each way for an unaccompanied minor, up from $50 each way for a nonstop flight.

Such fees and eligibility vary by the child's age and flight details (time, connections), says Parsons. Most airlines, for example, will not allow minors to fly out late in the day or schedule connecting flights that are too close together.

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Roomy Seats

Airlines are moving to smaller aircraft to cut fuel costs and fill flights, which means extra space is a precious and pricey commodity, says Banas. If you want to stretch out, be prepared to pay. Northwest charges $5 to $35 for seats with extra legroom, including exit rows and aisles. United asks fliers to pay a $349 annual membership fee for its Economy Plus Access program, which provides seats with up to five inches of additional legroom on every flight. JetBlue is the latest to try on seating fees. As of April 1, it began charging $10 to $20 extra (depending on flight length) for seats with an extra four inches of leg room.

A few airlines also charge for giving travelers a choice in seats. AirTran charges $6 per person if you want to choose your seat prior to check-in, while American charges $5 if you buy your fare through a third party and call to request a specific seat before check-in.

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