Baggy/bulky clothing is the only practical recommendation I know of here. That and leaving the house only when absolutely necessary were my pre-binding coping mechanisms. At least in a huge bulky coat, even if misgendering happened, it minimized the feeling that the whole world was looking at my chest (I know they weren't really, but it sure felt that way).
If it helps at all, there is probably no conscious thought process going on when people do this. That doesn't make it less rude or creepy, but the fact is most people are used to a subconscious, split-second assumption of the pants-contents of everyone around them based on a quick collection of cues. It doesn't occur to them that it's none of their business, or that it's impossible to know anyway between trans people, gender non-conforming cis people, and people with intersex conditions. So when they see someone they can't feel sure about "guessing", they instinctively, subconsciously start looking for more cues.