I'm no philosophy major or anything, but much of what I do with my politics major consists of political philosophy, and I can tell you a successful paper in one of those is one with bulletproof logic. Take the argument from point A to point B and point C in a deliberate and orderly way, and never lose track of the sequential chain; it's imperative you show how each part leads to the next. And I don't mean that it can't be refuted; I mean you raise the most significant counterarguments and deal with those accordingly, even if that means acknowledging you don't have a perfect answer (a professor once told me it's even permissible to come right out and say "I don't know").