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."

It's really easy. There you go.