Make sure your complaint has a focus on something the airline is really doing wrong. If your flight is delayed or canceled, that, of itself, is not a reason for a complaint; stuff happens. But if the airline fails to keep you informed, or if it does not provide appropriate assistance and compensation, or if its rules treat you unfairly, then you have a legitimate gripe. When an airline delays your trip substantially, existing European and proposed new Canadian rules require that the airline provide compensation -- real euros or dollars, not just blather. So far, in the United States, the only compensation specified by rules is in the case of bumping due to overbooking and baggage loss or damage. Anything else you might get depends on what an airline voluntarily puts in its contract of carriage, and those contracts are notoriously one-sided.