There are 5 Golden Rules......
1) Nothing is ever 100% secure.
2) Security is 90% PR: it's more about making the public feel safe (and the troublemakers feel threatened) than about the actuality of the situation.
3) The success of a security regime is ultimately judged more on its crisis response (and its subsequent PR handling of events) than on the effectiveness of its precautions.
4) Security is always a trade-off between minimizing risk and minimizing inconvenience.
5) Nothing is ever 100% secure.
I suppose number 2 is the most controversial (but it's not intended to devalue the importance of the other 10%, the more concrete security measures). I'd suggest number 4 is the most important. Number 3 is the one that I really worry about here in Beijing.