Know the area and choose clothing that is the "same" as the majority of people on the street. Most city dwellers and workers don't really pay attention to the "average Joe" on the street. But someone who stands out will attract attention.

So, if you are in a business district and most everyone is in suits, you need a suit and briefcase. If in an area with lots of construction/factory workers, dress like they do.

The closest to a "universally ignored" outfit I can think of would be something like or identical to one worn by the drivers of the major delivery services. They go everywhere and no one pays attention.

Military "urban" camo will make your stand out: everyone is paranoid and will spot you in a second, and remember you.