oHere is how I've written most of my NSpec tests, I usually start a context out with an act.  This dry's up the specification.  If I find myself doing to many "acts", I end up breaking it up into separate specifications.

    class describe_JavaScriptBundle : nspec
    {
        JavaScriptBundle bundle;

        void AddTest1JS()
        {
            bundle.Add("test.js");
        }

        void before_each()
        {
            bundle = Bundle.JavaScript();
        }
        
        void adding_a_bundle()
        {
            context["no files added"] = () =>
            {
                it["GroupBundle contains default key"] = () =>
                    bundle.GroupBundles.ContainsKey("default").is_true();

                it["has no Assets"] () => 
                    bundle.GroupBundles["default"].Assets.Count.Is(0);
            };
            
            context["adding one file"] = () =>
            {
            	act = () => AddTest1JS();
            	
            	it["contains the file"] = () => 
                    bundle.GroupBundles["default"].Assets.Count.Is(1);
            	
            	context["adding the same file again"] = () =>
            	{
            	    act = () => AddTest1JS();
            	    
            	    it["should still only contain one file"] = () => 
                        bundle.GroupBundles["default"].Assets.Count.Is(1);
            	};
            };
        }
    }