Install electromagnets in each seat and issue steel thongs to all passengers. Then refuse entry to anyone who doesn't set off the metal detector.
House bomb-sniffing dogs in the cargo hold. Would also boost burgeoning luggage cleaning industry.
Add ejector seats to cockpit. Remote controlled, of course.
Computerized passenger-matching places obese person, crying baby, or chatty traveler in every other seat, thus disrupting concentration of would-be hijackers.
Install cabin-length carpet and feed end under special slot in cockpit door. Yank in case of hijacking.
Replace barf bags with slingshots. Serve walnuts.
Replace warning chime with remote-triggered tape of Britney Spears singing with William Shatner. Set volume at "11."
Install trap door outside cockpit door. Accompanying ACME anvil optional.