The one thing travellers are doing wrong after they've booked a flight

Once you've booked a flight, you probably forget about your reservation until it's time to check-in.

Frequent flier and points expert Gilbert Ott - who offers people advice on flying for cheap, and sometimes for free, on the air miles site God Save the Points - thinks you're making a huge mistake by not paying attention to your booking.

"People generally book then they tuck [their itinerary] away, then the day they fly they pull it out again," he told Business Insider. "I say, look out for those emails [from the airline], actually read the things that they send to you, and, if you see anything you can use to your advantage, do it."

Airlines frequently make changes to flights in the lead-up to their departure, from the time and date they take off to the aircraft used for the flight. Passengers who ignore these changes are missing an opportunity to customise their flight schedule for free, and possibly to even get a complimentary seat upgrade, according to Ott.

"People are really bad at what I call 'gardening the reservation,' which is to look for changes that they can use to their advantage," he told Business Insider. "When an airline changes the schedule, which they do very often, you are generally entitled to some leeway."

Ott said there are a couple of ways that travellers "can turn a schedule change into something positive."

For example, if there's space on another flight, airlines will often allow passengers who say they are inconvenienced by the change to switch to an earlier or later flight for free, he said.

Clever fliers could also get a free upgrade this way, by looking to see if there's an earlier or later flight with empty seats in business or first class available.

Ott recommends doing this after you've booked even if you don't have a schedule change, to see if there are any vacant seats on your existing flight - though your chances of getting bumped up to the class above will be considerably higher if you have air miles to bargain with.

Travellers often believe they're at the mercy of the airline they are flying with, and while that's true to some extent - after all, they rely on the carrier to transport them from A to B - Ott says that passengers can reclaim some control over their booking when unexpected changes are made.

Even a change of aircraft can be enough to give passengers more pull with an airline.

On a recent trip, Ott said he used a change of aircraft to adapt the destination of his flight connection for free.

"I wanted to fly the [Airbus] A380 so I booked the A380 going both ways then [the airline] change[d] it so they didn't have the A380," he said.

When he called the airline, Ott complained that the change of aircraft was "unacceptable" and was able to switch his flight connection to go through Munich - on an Airbus A380 - rather than Pisa, the original destination which required flying on a different plane.

"Anybody can do this - you might have to argue a little bit but they'll give it to you eventually," he said. "Some people say, 'If it's not Boeing, I'm not going.'"