American Airlines ( AMR) will soon charge us an additional $30 for checking a bag on a round-trip flight. Why not just raise its fares as fuel prices rise and spare us a more cramped cabin?Because--let's face it--a good chunk of us go online, find the lowest fare on a flight aggregator like Orbitz ( OWW) or Expedia ( EXPE), and buy our ticket. Forget the airline's record with most on-time flights. Who cares if there's in-flight entertainment? We see a flight from Milwaukee to New York for $69, we're buying it. For airlines, fees are a way to game the system. Sure, one airline may charge $69 for its base fare, but tack on baggage fees, reservation fees and the cost of peanuts. Soon, you may be paying more than you would for a flight that, at first glance, looks more expensive. So how do you avoid being taken? Here's a rundown of questions to answer before you assume the ticket with the lowest fare is going to end up costing you the least. Fees apply to domestic flights, and many can be avoided if you belong to an elite travel club, pay full fare or buy unrestricted tickets. But for those of us looking for a deal, here's a rundown of how to get the best deal. How much is the first bag going to cost me? Alright, so only American Airlines has announced plans to charge for the first bag of stowed luggage. But after June 15, it'll cost most of us an additional $30 if the bag's traveling round-trip. If you're thinking, "I'll just pack my one carry-on bag extra tight," remember, everyone else is thinking the same thing. Even if you can cram all your stuff into the one bag, you might want to consider shelling out a few extra bucks for a flight where each passenger isn't desperately trying to carry as much on board as possible. What will additional bags run me? Even if you head out with only one bag, many of us have a way of accumulating knick-knacks along the way. So if you're likely to go crazy buying Kachina dolls on your trip to Santa Fe, you might want to know ahead of time what bringing back an extra sack full of crafts is going to cost you. Virtually everyone is charging $25 each way for a second piece of checked baggage. Notable exceptions: A second bag is free on Southwest Airlines ( LUV), and JetBlue ( JBLU) will charge only $20 starting June 1 (currently it's free). And AirTran ( AAI) charges $20 for your second checked bag unless you pay using Online Check-In, where the fee is cut in half. Are my bags overweight? Anyone thinking they can game the system with that huge piece of luggage in their closet, think again. Checked luggage has both size and weight limitations. Typically, you're limited to 50 pounds per piece of checked luggage and the length, width and height can't exceed 62 inches. If your luggage weighs more than 50 pounds but no more than 70 pounds, fees are generally in the $50 range. But Southwest and AirTran charge significantly less: $25 and $29 respectively. And Delta at $80 and United at $100, charge more. Luggage weighing more than 70 pounds may not be accepted by all airlines. Those that do generally charge $100 for bags up to 100 pounds. The outliers are Southwestern (which charges $50) Airtran ($69) and (on the high side) Delta ( DAL) ($150).