I'm not quite sure how to bring it about, being rather ignorant of naval warfare and completely ignorant of the German (and British) naval buildups and dispositions, but a more spirited and successful German naval campaign in the North Sea (or, conversely, a more flaccid and conservative British naval campaign; I'm not picky) would weaken the British blockade. This would have two effects: it would eliminate the necessity of German unrestricted submarine warfare, and thus keep a certain neutral power neutral; and more importantly it would allow Germany to import food more easily, thus maintaining morale and productivity on the home front and steeling it more for the long grind of attrition.