Have you noticed that padding on airline seats is getting thinner? Back in the days of the Lockheed Constellation and the DC-6, seats used to be less punishing -- they were more like La-Z-Boys. But don't expect to find cushy swivel chairs on your next economy-class flight. Today, in order to save fuel and squeeze in more passengers, the padding has been minimized -- and, as we all know, the rows of seats have been placed closer together. And seats may get even thinner.

Now, more than ever, upgrading can be a trip-saver. Here are eight ways to help make your next flight more comfortable:

1. Buy a seat with extra legroom. JetBlue, Delta, United and other airlines will sell you a few extra inches of legroom at the front of the economy cabin or at exit rows for a reasonable upgrade fee. We think this is money well spent. JetBlue charges as little as $10 extra for seats with 38 inches between rows, and has more leg room (34 inches between rows, rather than the 31-32 inches typical on some airlines) even if you don't pay more.

2. Buy a cheap business class seat on a discounter. Airtran and Spirit sell roomier business class seats for far less than most other airlines. Airtran will let you upgrade to business from any fare at the airport on a first-come, first served basis for $49-$99 per flight segment (that's one take off and one landing), or from higher economy fares in advance; Spirit calls their business class "the Big Front Seat" and fares are often less than other airlines' economy fares, especially if you're a member of their $9 Fare Club.   

Virgin America offers last minute upgrades (4 hours before flight time) to its luxe first class cabin for between $70 and $270 each way depending on the length of the flight, and US Airways has a similar program called "GoUpgrades" allowing passengers to upgrade from economy class to first for $50 to $500 each way, depending on flight length, 24 hours or fewer before flight time. You can upgrade by phone or at the airport, and international flights are included. See this chart for more details about these and other programs. And United has long had its Economy Plus option, offering "up to 5 extra inches of legroom" in coach, starting at $9 per flight for shorter hops and going up to, say, $109 each way on a LA to Tokyo flight. There's also a $425 per year "annual option" giving you unlimited upgrades to the roomier seats, subject to availability of course. Other airlines may offer spontaneous upgrades at the airport, so be sure to ask about them at check in. You never know what you might snag.

3. Look for Y-UP and Q-UP fares. These economy class fares, for domestic travel only, can be upgraded for free to business or first class, but they're not dirt cheap, aren't fully refundable and come with other restrictions. Buy them online (e.g., search for "all types" of business class fares on Travelocity), by phone from your airline, or through travel agents.

4. Choose planes with more legroom. Not all aircraft are created equal. Check out the "seat pitch" data at Seatguru.com and book on an aircraft on which seat rows are spaced further apart. Doing so can earn you as much as two inches of extra legroom.

5. Buy international business and first class from consolidators. Ticket sellers such as 1stair.net and planetamex.com sell premium cabin fares at considerable discounts.

6. Use miles to upgrade. This is one of the highest-value ways you can spend your miles. Upgrading a $400 fare on United from New York to LA to a $2000 business class fare for 30,000 miles is a better deal than spending those miles on a $400 fare. Unfortunately, many airlines now charge miles plus cash to upgrade ( see chart.)

7. Be loyal. Upper-tier frequent flyer program members get free upgrades, priority access to exit row seating at no charge, and other perks on many airlines, so stick with one airline, fly frequently, and attain "premier" status.