A survey a few years ago found that about 40% of vacationers financed their travel plans with plastic -- and more than half didn't pay off the tab right away.

We don't want you to stay at home and mope, but if you can't afford to pay cash for that cruise/camping trip/Civil War reenactment, ask yourself if you're willing to pay double for it. Because that's what it'll cost if you pay just the minimum amount due on your credit card.

Given that travel is one of the biggest budget line items for many folks, shopping around for savings is worth the time. With the average family spending more than $1,600 for their summer vacation, according to a survey by Visa, trimming costs by just 15% amounts to $240 in savings -- and that's just for the summer trip.

Below are some general tips on booking the lowest-cost airfare. We've hit the high points, but for much more nuanced advice, check out the Fool's Best Travel Spots/Tips board for tips from seasoned travelers and sightseeing advice from locals.

Five ways to catch a cheaper flight
Everyone knows that advance planning is everything when it comes to travel. Nowhere is the savings more significant than when shopping for airfare. The bonus to arranging your getaway now is that you can solidify your plans before all the aisle seats are taken (and have something to motivate the kids to do their chores). Here are some websites that will help you get there -- wherever that may be -- for less.

Track fares for a while. Many websites will keep an eye on airfare costs and alert you to the best deals based on your travel criteria. Check out cFares.com to search airlines, travel wholesalers, and agencies (for a membership fee, it also offers a name-your-price feature); and Airfarewatchdog.com for cheap flights before they hit the major travel portals.

Do comparison-shopping diligence. Make sure a deal is really all that by comparing your quotes with those offered directly by the airline. In addition to the biggie travel sites like Expedia (NASDAQ:EXPE) and Orbitz (NYSE:OWW), there's a plentiful list of aggregators to choose from like Travelocity.com, Hotels.com, and Travelzoo, which delivers its deals through its website and its Top 20 email. Other aggregator sites include Kayak.com, SideStep.com, Yapta.com, 1800FlyEurope.com, and I'mWillingToRideInCargo.com (kidding on that last one).

You might encounter some fares that require you to pick up the phone and contact the agent directly. Do it -- that's how I got a direct flight to Paris for half of what it cost on another website. Also, there's always Priceline.com (NASDAQ:PCLN) and its name-your-own-price policy.

When browsing, keep in mind that some airlines, like Southwest Airlines (NYSE:LUV), might not even offer their listings through larger aggregator sites. And once you've selected a flight from an airline, double-check at the airline's own website to see if you can avoid booking fees or can score a lower price on the same flight.

Pay with points. Planners -- those who book their travel months in advance -- will get the most out of their points and frequent-flier miles. These days the restrictions on rewards redemption are plentiful. (Check out mouseprint.org to see how the fine print might affect your travel and InsideFlyer.com for the most recent news about rewards travel.) A good rule of thumb is to pay cash for tickets that cost $250 or less instead of wasting points (unless they are due to expire). You may save more by using your points to pay for a hotel room. And if you're just short of qualifying for a freebie, check out Points.com to see whether you can swap or augment your rewards kitty. Airline loyalty programs don't always count by the mile. For example, JetBlue (NASDAQ:JBLU) offers its TrueBlue program, where 100 accumulated points equal a free round-trip flight, while Southwest's Rapid Rewards program serves up a free flight after eight flights within a 24-month period. Be sure to find the best program to suit your travel preferences and habits.

Wing it with a last-minute deal. Hotels and airlines hate empty rooms and seats. When it gets down to the wire, they'll settle for a song. But you have to act quickly: The deals are limited and they can go fast. Sign up for weekly email alerts with a few different travel websites. If you're already packed, lastminute.com searches for deals on airfare, car rentals, and hotel rooms that can be booked as little as three hours before takeoff.

Check out my hometown. You might have your eye on Hawaii, but unless you're prepared to pay more than $673 per day for a family of four, according to AAA's 2008 vacation costs survey, it may not be in the cards. While my hometown of Lawrence, Kan., may not be as brag-worthy, at less than $200 a day for a family of four, it still beats the high cost of cities like New York (around $606) or Miami Beach ($370).

More ways to save ...

  • Use this travel pro's tricks for getting the most from your frequent-flier miles: According to Consumer Reports, 75% of airline reward miles go unused each year. It's no wonder, given the plentiful and ever-changing rewards-card restrictions. Read "Rack Up the Rewards" for seven tips on getting the most from your miles from a fellow Fool and loyalty travel guru.
  • Try the bus: Greyhound isn't the only service out there. Here on the East Coast there are "commuter buses" that run from Washington, D.C. to New York and beyond for ridiculously cheap fares. Some buses are even equipped with free Wi-Fi so you can kill time on the road.
  • Hit your wholesale club for cheap tickets: Many Costco (NASDAQ:COST) and Sam's Club locations have travel departments where you can score a good deal from time to time.
  • Some quality assurance for the bargain hunter: If you're hunting for bargains and aren't sure of the hotel, check in with TripAdvisor. You'll get the scoop about important things to consider like location, cleanliness, and safety. Likewise, SeatGuru.com offers additional information about airlines and specific flights.

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