First, we need to accept that no preventative action can stop all mass killers. Second, we need to accept that in at least some cases, there are preventative actions which can make a positive difference without turning schools into mini-police states.
I believe that the best preventative actions come from identifying the thought processes and actions which are the stepping stones toward mass violence and having the power to act on multiple pieces of information which individually may not be warnings of any criminal behavior.
Because there have been reports that Cho Seung-Hui was bullied some people have made bullying the cause. I disagree. Bullying isn't good and should never be ignored or viewed as harmless, but other people were likely bullied far more than Cho Seung-Hui and have never assaulted anyone. It isn't a matter of stimulus and response but stimulus -- choice -- response.
I've seen calls by some to have all mentally ill students removed from school, but that would harm plenty of students who pose no risk to others and it would give students a practical reason not to get help when they need it most. This might increase school violence rather than reduce it.
It may seem like an unneeded step, but we need to communicate individually and as a society that responding to bad or humiliating situations with violence is wrong and that there other acceptable options besides violence. Then we need to enhance those options so they will be used when they are needed.
For many mass suicide killers I believe they are attempting to use their violence to turn themselves into anti-heroes. Anti-violence initiatives should help communicate that there is nothing heroic or grand about planning mass murder.