This year, I did a lot of travel for my job but never signed up to get frequent flyer miles. The trips were on a variety of airlines, and they cycled around so fast, I just didn't get around to earning miles. I traveled infrequently before and just did not know the drill. I have all my old tickets and have a slow period coming, so can I get the earned miles after the trips?
Your chances of receiving mileage credit for flights taken before you enrolled in a program (generally referred to as retroactive credit) depend on two factors: 1) the policy of the airline that operates the program you neglected to join, and 2) how long ago the trip took place.
As far as policies go, a check of the largest airlines' programs reveals a range. As you might expect, the dominant approach is to simply deny credit for flights taken prior to enrollment. American expresses this attitude quite succinctly in the AAdvantage program's terms and conditions: "Transactions prior to your enrollment date are not eligible for mileage credit." Northwest and United, among others, follow the same policy.
At the other end of the spectrum, Delta will credit you for flights taken within 30 days of signing up for the SkyMiles program. To apply for retroactive credit, use the form on the "Request Mileage Credit" page of Delta's website.
Occupying the middle ground is Continental, which will award credit retroactively, but charges a fee to do so. For $25, OnePass members can request mileage credit for flight activity on Continental, Continental Express, Continental Micronesia, Continental Connection (operated by Cape Air, CommutAir, Gulfstream International Airlines, and SkyWest Airlines), and Copa Airlines during the six months prior to your enrollment in the program. (Given the airlines' financial difficulties and the resulting proliferation of fees, I predict that other carriers will adopt the Continental model in the not-too-distant future.)
So depending on which airlines you flew, and how long ago, you may or may not be able to salvage the miles.
In the future, there's no reason to expose yourself to such uncertainty. It only takes a few minutes to sign up for program membership on the airlines' websites. And it's free.