I am brand new to this site and know basically nothing about frequent flyer miles. I am about to start traveling more for fun, especially now that I've realized how inexpensive flights can be (thanks to this website.) I usually fly with US Airway since I fly out of PHL. If I sign up with their program can I get miles retroactively for flights I've flown in the past? Also....should I even use their program? Or is there someone else I should consider. Any help would be much appreciated.