Hopper’s signature rabbit may seem a bit childish at first, but this bunny knows its stuff. The flexibility feature is especially useful: put in your home city and desired destination and Hopper will map out the entire year for you with color-coded prices. The dates are shown as month-long calendars, with colored boxes telling you when prices fall into certain categories, like less than $300 or +$500. The app-only service also predicts whether or not the price for your chosen dates may drop or not (they monitor up to 15 billion flight prices per day) and allows you to “Watch the Trip” —  a feature where Hopper will send you alerts if the price changes. If you know you need a vacation, but are flexible on your destination or dates, you can also set alerts for the app to tell you when there are destinations deals. This app is free to download. 
Most cash bookings can be upgraded with Avios either at the time of booking or later on, depending on availability. Only the lowest economy (World Traveller) fares (Q, O and G) cannot be upgraded with Avios at any time. There must be reward flight availability in the next cabin for you to be able to upgrade to it using Avios. You won’t be able to upgrade with Avios at the airport or on board.
The best time to check in with airports is typically around 24 hours before your departure. You can also sign up for flight alerts, which allow you to program your flight or preferred flight for an upgrade. The alert will automatically search for open availability and let you know as soon as it’s open, giving you the best chance to scoot on up to the front.
Travelocity.com: Travelocity, as far as we know, was the first travel agency site to offer a fare watch system. Once upon a time, it tracked both international and domestic fares, but now only covers domestic/Canadian deals. To use it, go to the "Flights" tab and then to "Low Fare Alert" and then click on "Get email alerts" in the upper righthand corner under "Tools". You can sign up for alerts on up to five routes. On the Low Fare Alert page you'll also see a list of fares, from any given city, that have gone down in price from the previous day. Travelocity searches fares just once a day, however, compared to Farecompare's three times a day Monday-Friday.

Here’s where the power of ExpertFlyer comes into play. Let’s say that I wasn’t interested in crossing the Atlantic in economy. Instead of coming back to the site multiple times in the weeks and days leading up to my desired flight, I can actually set up a flight alert by clicking on the exclamation point icon (red arrow in the above screenshot). That would open up a window like this:
So, for example, if you really like to take United's morning nonstop between New York and LA, you can track just that flight to see if the price drops to a level you specify, and you'll also get an alert if the price drops between the time you buy and when you fly. Yapta requires that you download an applet, and only works with Internet Explorer 6 or 7, which means it doesn't work on Macs, although a Mac/Firefox version is in the works. And it does not (yet) track fares on all airlines — just 11 so far, and as usual Southwest isn't one of them.
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Number 2 is that you don’t even need to know where you want to go. Based solely on your fixed dates or a broad time period, the Explore option shows you where is cheapest to fly, which is a perfect tool for the early stages of your trips. I use it this feature a lot when planning a trip itinerary or choosing which hub city should I adopt in a multi-city trip.
A price alert tracks the price of a specific route or flight. When the price changes, you’ll be notified via email or push notification if the price went up or down (and by how much). Price alerts are completely free, can be stopped at any moment and you can have multiple set up at once. They are hands-down one of the best ways to find cheap flights, fast.
If you get through security and have not been able to upgrade yet, stop at the gate desk to ask to put be on a list if upgrades become available; the practice of overbooking exists in part because folks with refundable airfares often decide not to fly at the very last minute, and seats can come loose almost right up to the point the aircraft doors close.
Charge for a change to the flight date up to 30 minutes before departure before web check-in closes RBK €70 / £61 / CHF 81 / $82 / CZK 1,800 / SEK 717 / NOK 667 / PLN 301 / HUF 22,332 (plus difference in current flight price) €90 / £79 / CHF 104 / $105 / CZK 2,314 / SEK 922 / NOK 858 / PLN 387 / HUF 28,712 / AED 424 / THB 3,700 (plus difference in current flight price)
A price alert tracks the price of a specific route or flight. When the price changes, you’ll be notified via email or push notification if the price went up or down (and by how much). Price alerts are completely free, can be stopped at any moment and you can have multiple set up at once. They are hands-down one of the best ways to find cheap flights, fast.
Orbitz.com: This well-known online travel agency is best known for its fare search capabilities (like Farecast, it runs on ITA Software). But Orbitz also offers fare alerts with its "Deal Detector" (find it under the heading in the lower left of the home page under "Orbitz deals — delivered to you"). Once there, fill in the departure and arrival airports (you can also opt to get alerts for cities within 80 miles of your main choice), choose whether it's a weekend trip or not, specify your dates of travel, and decide whether you're flexible plus or minus 1 to 3 days. Then set your target price and whether you prefer nonstops. You can only sign up for one deal at a time, and again, Orbitz doesn't list every possible airline.
A price alert tracks the price of a specific route or flight. When the price changes, you’ll be notified via email or push notification if the price went up or down (and by how much). Price alerts are completely free, can be stopped at any moment and you can have multiple set up at once. They are hands-down one of the best ways to find cheap flights, fast.
I’ve included these two together in this instance as the startup team behind Yapta jumped on board with KAYAK back in February of 2010, and the two websites share a crossover of features and a similar design. The combination of Yapta and KAYAK probably offers the most powerful combination of features for tracking flight costs, but it can be a touch finicky to use and is not as intuitive as Google Flights.
Yapta.com: The newest entrant in this burgeoning field. There are two things you can do here: first, if you've already bought a fare, you can enter the flight locator number and then you'll get an alert if the fare has gone down. (Some airlines will refund the entire fare difference in the form of a voucher good for future travel within a year; others deduct $25 to over $300, depending on the fare and route.) But there's also a "Tagger" feature where you can track fares on individual flights, not just routes between two cities.
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