Credit cards which earn frequent flyer points is a popular way to earn frequent flyer points. You can receive thousands of points on everyday spend. And, many of these cards offer generous signup bonuses! Compare to find the credit card that best suits your needs.
You can credit each flight to a different program if you like. Easiest is to credit each airline to a selected program. You can change the program for a particular flight right up until boarding time - even if you are already checked-in and have a boarding pass. Just make sure the print you a new boarding pass so you have proof if needed to send to the airline for crediting later.