Often, various airlines provide special rates on certain flights to popular cities. If you find them, don't waste any time buying your seats, as the offering time can be as little as 24 hours or less. There will probably be restrictions on when you can depart.
There is a website called SkipLagged, that lists numerous flights every day at very discounted prices. For example, it has a special flight that it found for flying from New York City JFK airport to London for only $437.
Just remember, when you see these deals, grab them when you can.