I forgot to put my frequent flyer number in my reservation. Is there a way to still get the miles?

Yes, credit can usually be obtained after the fact – so long as you have your boarding pass. Simply go to the frequent flyer portion of the airline’s website and follow the instructions for missing credit, which usually involves entering the flights and dates and adding a scanned image of the boarding pass(es). The ability on most airlines to have an electronic boarding pass can be a big help as it is easier to hang on to than a paper document. If you don’t have either an electronic or paper boarding pass it may still be possible to get credit, depending on the airline.