In white South Africa sanctions worked because there was a body of business interests and voters who were hurt by sanctions and who could lobby their government. Public opinion was a political actor. That is not true in Baghdad and only slightly true in Belgrade. Sanctions simply impoverish a nation, creating precisely the conditions in which an alternative political class cannot prosper. In Iraq, they have become a form of collaboration with the very dictator they are meant to remove.
Sanctions can only work, it seems, where there is a public opinion to influence.
That may not be much of a rule, but it could be a start. The US has moved toward that idea and it is a good one, targetting the people who matter rather than everyone. As for the rest of the rulebook, the demand for UN consensus before international action is a noble ideal and maybe a good starting point. Whatever rules we devise we need to get to work quickly. If our world is becoming a global village we have to decide how to police it - and soon.