Such early actions almost never exist. Perhaps mediation efforts between Israel and the Palestinians is a rare example of it. In Bosnia the international community has acted, but only after a very great deal of violence and tremendous human suffering. Kosovo may be seen as a further evolution of the violence by the former Yugoslavia, first against Slovenia, then against Croatia, then in Bosnia, before Kosovo followed. What kinds of actions are needed.
Preventive actions can take many forms.
Neutral parties can help groups overcome past antagonism. Nations and the international community can help countries with democratization that might lead to changes in the elements of a culture that enhance the danger of mass killing or genocide. As discrimination and violence against a victim group begins, nations can make aid contingent on changes in such behavior. They can offer help with mediation and conflict resolution. They can provide support for economic development.