You may tell your airline if you prefer an aisle seat to a window seat or if you'd rather have chicken than pasta.
Some things, however, like the fact that you pick all the cashews from the mixed nuts, or that you always drink a gin and tonic in the airline lounge, you might not.
Lately, though, airlines have a way of knowing your preferences, even when you don't tell them. The culprit? Social media, of course.
Recently Qantas started a tryout to monitor digital conversations on social-media sites like Twitter or on Facebook.
The technology comes from Australian start-up Local Measure, and it allows corporate clients to search not by keyword but by GPS coordinates.
As a result, when a customer posts a picture on Instagram (even one without a caption) from one of ten Qantas airport lounges, the customer care team is alerted immediately.
"It allows us to pinpoint customer feedback to a specific location. If someone raises an issue, it gives our lounge staff the ability to step in and resolve the problem," says Jo Boundy, head of digital communication at Qantas.
If a customer groans on Twitter about a long line at the coffee counter or a broken printer in the business lounge, Qantas can respond in real time.
Social media is a popular medium for KLM customers and led to customer-made city guides.
According to Jonathan Barouch, chief executive at Local Measure, other airlines will be soon introducing the technology to their lounges as well, allowing the companies not only to respond to problems, but to anticipate them as well.
"It helps our clients better understand what people are thinking about their experience," he says.
For example, Barouch says, a customer recently tweeted a picture of cereals on offer at the Qantas first-class lounge and noted that his favorite was absent. The next time he visited the lounge, the cereal was there.
"Qantas knew how to surprise and delight the consumer. It's all little things, but they do add up," he says.
Because of the speed with which a staff member can respond on Twitter or Facebook, social media has quickly become a favored platform for airlines' customer service.
KLM Royal Dutch Airlines employs 130 people to monitor different social-media platforms, including VKontakte, a social networking website popular among Russian-speaking users, and Sina Weibo, the Chinese equivalent of Twitter.
Social media has become such a popular medium for KLM customers that the company introduced an estimated service response time to their Twitter account, even though the ability to do so isn't built into Twitter (KLM works around this by updating the background image every five minutes).
For the most part, response times hover around one hour.
"We decided to put our money where our mouth is and show our real-time performance," says Martijn van der Zee, senior vice president of e-commerce for Air France-KLM.
Reflecting how attuned the company is to their online followers and customers, their suggestions help KLM create new products and services, including one introduced last week that allows non-passengers to purchase upgrades and gifts for friends and family traveling with the airline.
Other ideas developed from social media are destination guides based on cabin-crew favorite spots, and a service that allows staff members sweep planes for forgotten wallets, tablets and mobile phones, and contact the customers (sometimes before they realize their item is missing).
"If these were suggested to me four years ago, I might have said, 'These aren't relevant,' but now we know that they are, because we get messages coming in every day telling us they are," says van der Zee.
"We have 30,000 interactions a week. That's a lot of input, and it's motivated us to keep pushing the envelope."