Tips for finding lower fares
The price of a ticket moves on three things you control: which days you fly, which airports you use, and how long you wait. Everything else is noise.
Move your dates before you do anything else
A single day either side of your trip is worth more than every other trick on this page combined. Airlines price by demand, and demand is concentrated on Friday and Sunday. Tuesday and Wednesday departures are cheaper for the same reason the same hotel room is cheaper on a Monday.
Every serious search site has a calendar or matrix view that shows a month of prices at once. Use it first. If you have found the cheapest fare for your exact dates without checking the surrounding week, you have not finished looking.
Check the airport next door
Search by city, not by airport code. Most search sites will include nearby airports if you ask, and the gap can be large: London has six, New York has three, and the Bay Area has three that are all a train ride apart.
Two cautions. Count the cost and the time of getting from the cheap airport to where you are actually going, because a 90-minute bus can eat the saving. And a secondary airport at midnight is a different proposition from the same airport at noon.
Price one traveller, then book everyone
Airlines sell seats in fare buckets, and a bucket holds a fixed number of seats. Search for four people and the site quotes you the price of the fourth seat, applied to all four. Search for one and you see the real cheapest seat.
So price it as one passenger to know what is actually available, then book the group in one reservation if you want to sit together. If the difference is big and you do not mind splitting up, book separately.
Two one-way tickets are often cheaper than a return
On domestic routes in the US and on most low-cost carriers, a return ticket is just two one-ways added together, so you can mix airlines and take the cheaper half from each. On long-haul international routes the opposite is usually true and a return is much cheaper than two one-ways. Check both. It takes a minute.
Book, then keep watching
In the United States you have 24 hours to cancel any ticket for a full refund, as long as you booked at least seven days before departure. That is a federal rule, not an airline courtesy, and it applies to any flight to, from or within the US.
Use it. Book the good fare when you see it rather than agonising, then set a price alert and check again the next day. If it drops, cancel and rebook. After the 24 hours are up the same trick still sometimes works: if the fare falls a long way, some airlines will issue the difference as a credit if you ask.
What does not work
Incognito mode does not lower fares. The theory that airlines raise the price because they saw you search twice has been tested repeatedly and does not survive testing. Prices move because seats sell and because the airline is running its own algorithm, not because of your cookies.
There is no universal cheapest day to buy. Studies that name one are averaging thousands of routes into a number that applies to none of them. Watch your route rather than a headline.
Common questions
How far in advance should I book a flight?
For domestic trips, roughly one to three months out. For international, two to eight months. Those are the windows where prices are usually lowest, but the range matters more than the midpoint, and a good fare inside the window is worth taking rather than waiting for a better one.
Can I cancel a flight I just booked?
Yes, if the flight touches the United States and you booked at least seven days before departure. US rules give you 24 hours to cancel for a full refund. Outside that, it depends on the fare you bought.
Does clearing cookies make flights cheaper?
No. Fares change because seats sell and because airlines reprice constantly. Private browsing has no effect on the fare you are shown.
Is it cheaper to book two one-way tickets?
Often on short-haul and domestic routes, rarely on long-haul international ones. Price both before you book, since it costs nothing to check.