There is nothing worse than being on an overnight, long-haul international flight crammed into a 3 across seat configuration.  Try getting some sleep on that one!   However, what is much more comfortable is having the middle seat open. It lets you stretch out and relax. The amount of space with that middle seat open makes an extraordinary difference compared to having that seat occupied.

Icelandair, for instance, offers an economy class seat from North America to Iceland for $202, one way.  They also offer a "Comfort Economy" option where the middle seat is blocked off.   The "Comfort Economy" seat costs $562.

So, the strategy is to purchase your economy class seat (aisle or window) then an additional economy seat (middle) for another $202.  Voila! More room for you at a lower cost than buying a "Comfort Economy" seat.   The Comfort Economy seat does come with a few more perks than the economy seat, but the savings is even more attractive for purchasing economy seats if traveling with a partner.   Three economy seats costs $606 (with the middle seat open) where 2 Comfort Economy seats costs $1124.  That's a $518 savings which supercedes any perks the flyer may get for purchasing the Comfort Economy seats.

But hold on - it gets better.  When buying an empty seat, that "seat" is exempt from all or some taxes.   Since taxes are applied to a person, and there is no person in the middle seat, take some of the taxes off the cost of the purchase of the middle seat.  In the case of Icelandair, it comes to about another $30.

Let's look at another situation.  An economy R/T ticket between JFK to CDG (Paris) on American Airlines in January is priced at $457.   Instead of buying two business class tickets for $7114 ($3557 each), just buy out an entire economy row of 3 across for $1371 minus the taxes (up to $125) on the middle seat.

Please note that frequent flyer miles are normally not available for that empty seat and that it probably needs to be booked over the phone with the airline.  But in any case, this tip may can result in a much more comfortable flight, for a lot less money than you thought it would cost you.