I travel often. I travel to New York, Boston, Detroit, Los Angeles and San Francisco every other month for my job. But I always travel through different airlines, whatever flight is the cheapest. I may travel through one airline but not travel with them again for another 6 months or so. I wanna sign up for frequent flyer benefits but I don't know if I could since I never use the same airline. Is there a way to sign up with multiple airlines or do I have to use the same airline all the time?