source: smart travel
Get the flight miles you deserve
In order to get your flight miles credited, you need to add your frequent flyer number to your reservation. The good news is that you can accomplish this at several stages during the reservations and travel process, so if you miss out on one opportunity, you still have the chance to ensure you get all the miles coming to you from that trip.
Before your flight: The easiest and most fail-proof way to guarantee you'll get miles for your flight is to provide your frequent flyer number when you make your travel arrangements. If you book online, the reservations form almost always provides a space to enter your number, and if you book by phone or through a travel agent, you can simply tell them your number. If you forget, you can always call up your airline and have your number added to your reservation.
In the weeks leading up to your flight, you should also check the SmarterTravel.com Mile Finder or your airline's website to see if your flight is eligible for bonus miles. Many offers require registration prior to your flight; if you don't sign up, you won't get the extra miles. When checking, think about your flights as individual segments, especially if you have a stopover. Sometimes you can earn bonus miles for one leg of your trip, if there's a promotion between those two cities, rather than between your origin and destination cities.
On the day of travel: Check to see if your member number is printed on your boarding pass. If not, ask the gate agent to add it. Usually, if your number isn't on your ticket, it means that your number isn't attached to the reservation. It doesn't matter if you provided your number at reservations or not; you can always add it at the last minute. But if you do this, remember to check your ticket on your return flight as well. Sometimes the agent only adds the number to your current flight and forgets to add it to future travel.
Also, be sure to keep all flight documents (boarding passes, itineraries, etc.) until you notice that mileage has been credited to your account. Having documentation makes it much easier to get back missing credit after a flight.
After your flight: If you get to your destination or return home and realize that you forgot to add your number to your reservation or that for some reasons your mileage didn't post to your account, you still are eligible to collect miles for your flight. Just be careful because time is running out. All six of the major U.S. airlines state in their program rules that they will consider claims for mileage adjustments within six months or a year of the original flight activity. Airlines reserve the right to request proof of activity, such as an e-ticket itinerary, boarding pass, or ticket stub; and Continental even has a specific "Mileage Request" form for you to fill out. Delta states clearly that only written requests will be accepted; verbal requests made by phone won't get you far with most airlines.