in no particular order:
don't assume that all americans celebrate christmas.
don't be offended when someone suggests that you might want to consider that not all americans celebrate christmas.
don't snort or sigh when you hear someone suggesting that not all americans celebrate christmas.
understand that there are many cultures and religions in america.
consider that it's not appropriate to flaunt christmas in public environments without any regard for others' holy days.
love christmas, celebrate how you see fit, but don't think that everyone has to celebrate with you.
don't confuse the term 'american' with the term 'christian' nor the term 'christmas' with the term 'american'.