RULE #1: Always book your flight directly with the carrier(s) (United, Lufthansa, Virgin, etc.) and not thru 3rd party services such as Orbitz, Travelocity, etc. (the same concept applies to booking hotels). The primary reason is that if you’ve book directly with the carrier and something goes wrong, then you only need to deal with the carrier to try to straighten things out. On the other hand, for example, if you book a United flight thru Orbitz, and if something goes wrong, you may wind up in a situation where United tells you to talk to Orbitz, and vice versa. Not a fun place to be when you’re stuck at an airport in a foreign country trying to get somewhere.
Another reason to book directly with the carrier is that their prices are typically identical to those you can find on 3rd party sites.
That Being Said:There several sites I use when planning an trip (domestic or international) that help me find the best flights for the dollar, such as:
Orbitz.com – After having told you not to book on this type of site, I have to admit I always start with Orbitz when researching my flights. But once I see who’s offering the best fares/routes, I always book directly on that carrier’s site. Not all airlines have a relationship with Orbitz and other booking sites. For example, none of the Virgin airlines (Virgin Atlantic, Australia or America) sell thru consolidators like Orbitz. So you may want to check directly with any airlines you think might cover the route your trying to book.
Skyscanner.com – This site allows you to check almost every available flight/route in the world. I use this site when traveling within Europe to find the cheapest flights/routes (e.g., London to Milan, or Berlin to Mallorca). After finding the least expensive routes/carriers for my needs, I then go to the carrier’s site to book the ticket (see rule #1, above).
Too Good of a Deal?: There are a lot of budget carriers throughout Europe (i.e., Ryanair, easyJet, Air Berlin, etc.) that offer great prices. There are a couple things to consider however:
1) They tend to fly into/out of “2nd tier” airports. For example, flying from Dublin to London, Ryanair will likely fly you into Gattwick, London City or Stansted airports instead of Heathrow. I actually prefer to fly in/out of one of the smaller airports because Heathrow is very large and crowded and a pain to get through. But Gatwick, London City and Stansted are much smaller/easier and are served by the tube or train so getting into the city center is not a problem.
2) The price they advertise is generally not the price you’ll wind up paying. For example, I recently looked up a one-way flight on Ryanair from Dublin to London City airport. The base ticket price was €30.99. But if I wanted to check a small bag (up to 15kg, or 33lbs) add €25, and if I wanted to actually reserve a seat add €5.99 (for a “standard” seat, €10.99 for a “premium” seat). Want to get priority boarding, add €2.99. So without the priority boarding, and with a standard seat and small luggage, I’m up to €61.98 (or about $66) or about double the initial advertised price.
$66 is still pretty cheap to fly from Dublin to London, but it’s best to keep this in mind when comparing the cost of inter-European flights. In some cases, and “all-in” ticket from Aer Lingus (Ireland’s national carrier) might be the same price and a lot less hassle than Ryanair for getting to London. For extensive info on budget airlines, see Rick Steves' Europe.
Seatguru.com– This site has the seat configuration of just about every airline and flight out there. Simply put in your flight number and date, and it will display a seat map with comments on better/worst seats.
Note, in some cases the airline will have multiple configurations for the same flight (on different days), so check your itinerary on your carrier’s site to confirm which aircraft configuration is scheduled to fly on your date.
Also note, airlines sometimes switch planes between when you book the flight and the day of your flight, so it’s best to check back within a few weeks of your flights.