The major challenge here is that both sides were near their breaking points, economically, socially, and politically. Four years of total war meant a lot of proverbial seed corn got eaten. The US was in good shape, having just entered the war in 1917, but Russia had already collapsed by 1918, Austria was on the verge of collapsing (only held on as long as they did due to German support), and Germany started collapsing around the time of the ceasefire. And Britain and especially France might have only been a few months from collapsing had Germany and Austria not beaten them to it.
Two ideas for prolonging the war:
- Reduce the load on Germany's economy by weakening the British blockade. Maybe a political decision in Britain to not classify food and other civilian supplies as war contraband (at least not until very late in the war), or perhaps a POD involving increased German naval success to the degree that Britain can't or won't intercept German convoys through the North Sea. The latter would be difficult to do without knocking Britain out of the war entirely.
- A change of strategy for both sides, deemphasizing offensives in favor of letting the other side wear itself out attacking your defenses. I'm not sure how to accomplish this, but if this happens in 1915 or 1916, the cost of the war might fall to the point where it's sustainable for a few more years.