My idea, which I've mentioned on other forums, is that it isn't the bad guys we have to worry about, it's the good guys. Typically these guys that shoot up schools, movie theaters and the like aren't street criminals who spend their nights breaking into people's houses or dragging people into alleys. While after the fact, it's always easy to point to them and say I could have told you so, they had no crime history. They may have had a history of other things but not crime as we think of it.
So if the criminals will always weapons (not necessarily so), the answer is to keep the good guys from getting weapons like they've been using. But I can't imagine that going very far.
My wife's great-great-great-great-great grandfather (give or take a great) actually wrote the 2nd amendment. That is, he sort of thought of the idea. His name was George Mason. He was particularly interested in the part that says "well regulated." And he definately meant it to apply to a literal militia, which became the National Guard, although it continued to be called the militia into the 20th century. For some reason he was worried about private armies for some reason, though I don't think there was any precedence for that. What I absolutely don't believe they intended when that amendment was added was for women to carry concealed firearms. But I can't imagine that going very far today either.