A combination of:
- No Zimmerman Telegram
- No German USW
- Germans stroking Wilson's ego and feigning interest in his peace initiatives
I'd say all of these three would be a good way to keep the U.S. neutral. But especially the third point.
The Germans could try to use the United States as a intermediary in peace talks with the allied powers. It might even have worked. At this point war weariness was evident on both sides. Other threads have discussed this but honestly I don't see the war lasting much longer without revolutions breaking out in the UK, France, Italy, and Germany (witness how bad things were in France and Germany in OTL.)
A neutral intermediary like the U.S. could be asked to host peace talks to end the conflict. Not only do peace talks start early, the U.S. is kept out of the war, and the Germans can negotiate a peace treaty that is much more acceptable then the one that is imposed on them at Versailles.
Also, bear in mind that there were several large enclaves of German immigrants throughout the U.S. at this time which was one of the reasons President Wilson wanted to avoid getting involved in WWI. There was plenty of political capital to be gained by showing the U.S. as a peacemaker here.