Honestly, as a foreigner in a country often visited by Americans (Italy), just be yourself, polite and don't raise the volume of you voice in English when speaking to the natives (and yes, I have seen Italians do the same in the US - no, speaking Italian slower and louder will not help the lady in Macys understand you).
The bits about white sneakers or crocs are just basic style. Be comfortable but decent. Again, be polite and don't draw unwanted attention to yourself. It will be fine - Americans are usually liked - they are nice, good tippers and appreciative.