Soccer cheering etiquette in the USA is as easy as cheering for BYU.
You never cheer for your main rival - Mexico. People that do are in the same boat as people that wear that stupid BYU/Utah hat.
You cheer for your region's teams next - CONCACAF. You still never cheer for Mexico. But if Honduras is playing Spain, you should be cheering for Honduras.
You never cheer for the Alabamas, USCs, or other huge programs - Bye Brazil!
Make fun of and exaggerate your rival's negative stereotype. For example: "Every Mexico fan I've ever met has tried to throw a urine balloon bomb at me." (think of the West Concourse at RES stories that are told here on CB for reference)
The next one is the most important one:
There are always outside factors that are stopping the USMNT from achieving success. For example: "We would be winning WCs every year if only our best athletes played soccer". Or "FIFA has so much corruption that it is keeping the US down because we play clean."