Here is the easy answer. It is called......OTL.
But to answer your question, I think the following would be the easiest way to extend the war:
1. No massive offensives (Somme, Verdun, etc), just unending low level sniping from the trenches.
2. Germany would have to find ways to work around the blockade effects. Victory Gardens?
3. Russia bows out earlier than OTL, and stays out.
4. US stays out, which means no USW by Germany.
What am I missing?
Pretty reasonable. With NO USW by Germany Russia leaves a few months earlier, helps Austria and Turkey.
If we keep Falkenhyn in charge means no Hindenburg economic plan which made things worse for Germany OTL.
Germany keeps a trickle of merchant subs going to and from the USA bringing in strategic supplies (mainly real rubber, nickel)
Germany can not choose to do Verdun in 1916, do limited things in the east, Salonika or Italy, perhaps finishing off Salonika means Bulgaria has no active front or Italy is knocked out early.
Germany does less counterattacks during the Somme.
Britain/France rely on the blockade which doesn't quite work with Russia out/USA neutral.
With Russia out and USA neutral Germany doesn't have to risk a big offensive in the west.