Many of you have already said that the reasons these terrorist acts were so easily executed were the many flaws and lack of security within the American flight transportation system. I've thought of several measures which might possibly help from these acts from being reiterated again:
1) Make the door/wall separting the pilots cabin from the passenger cabin more secure - meaning locked, made of sturdy metal, etc. If this is inpenetrable, the pilots would have never been killed and the planes would have never been in the wrong hands for an even more devastating atrocity. Maybe a bulletproof window on this separator will also help, allowing pilots to see what, if anything, is going on behind them. A pilot leaving his cabin not only endangers everybody onboard, but allows hijackers to take control of the plane and wreak more havoc.
2) Post armed security guards aboard the plane. I believe it is ridiculous to see how there is no form of security besides the few flight attendants aboard the plane. I do not understand why our flight system makes it so tempting for anybody, even with a plastic knife, to be able to take over a plane by threatening the lives of all passengers.
These points might seem somewhat overwhelming for our current system, since it will need additional employees and maintenance for many planes. Whatever they may be, I bet the costs for these will never surpass the costs WTC atrocity.