How do you evolve & version an interface?

Question

Say you have an interface IFoo:

public interface IFoo {
    void Bar(string s);
    int Quux(object o);
}

In version 2 of your API, you need to add a method Glarg to this interface. How do you do so without breaking your existing API users and maintaining backwards compatibility? This is mainly aimed at .NET, but can apply to other frameworks & languages as well.