Off the top of my head:
Have the BEF active in the immediate aftermath of the Battle of the Marne, and advance into the gap between the German 1st and 2nd armies. This isolates the German 1st army, leaving the French with enough forces to both engage the German 1st Army, tying it down in fighting, and get forces around the flank, winning the race to the sea. Once strong forces are in behind the German forces, the supply lines are cut, and it's all over.
Ottoman Empire doesn't join in, or, better yet, joins in on the Entente side. Britain honours its agreement over ship sales, tosses a few bribes, and the southern supply route to Russia is open. This (a) delays problems in Russia, (b) places greater weight of forces against Austria-Hungarian and supports the Entente countries in the Balkans, (c) frees up British manpower that is no longer needed in Mesopotamia and Egypt and Gallipoli, (d) enables Russia to deploy more forces against Germany in the north, causing Germany to bleed faster.
Bring the hard winter of 1917/18 forward to 1916/17. By this point, the German agricultural system was already under severe strain (rationing, malnutrition, deaths from disease brought about by inadequate food supplies - all were happening as German food production fell and distribution collapsed). If the Turnip Winter happens a year early, it's probable that the country that falls apart in revolution will be Germany, not Russia.