This idea's been tossed around on this forum a few times before, but this gentleman's put some serious thought into it. He seems to stop short of suggesting that people be allowed to carry firearms, but still, it's a reasonably well thought out system:
http://www.americanthinker.com/2010/11/time_to_arm_airline_passengers.html
http://www.americanthinker.com/2010/11/time_to_arm_airline_passengers.html
Qualifications can be set as to age, state of health, physical capabilities, absence of a criminal record, provision of character references, education level, and pertinent background, such as experience in law enforcement, the military, martial arts, or other applicable areas. Volunteers could agree to place themselves under the command of flight crews and to accept any seat they are asked to occupy, in understanding that there could be situations in which the crew would deem it a proper precaution to have such a volunteer or volunteers seated in close proximity to a possible threat. They could also agree to stay alert for the duration of the flight.