How do I do this? Specifically the clothing part. How can I best plan ahead to not label myself as a clueless tourist? I imagine my American standard of dress is too casual for most places, but beyond generalizations like that, I'm clueless.

You may not be able to do a lot of "planning ahead" but when you're there you can look at how people dress and emulate that. In some places it might mean buying tighter fitting clothes than you might wear in the states (some places in Europe a lot of men wear much tighter clothing & style their hair more than the average heterosexual man would in North America), or getting flipflops instead of sneakers or vice versa, seeing what sort of bags people carry. I wouldn't think of the goal as being looking exactly like a local so much as not wearing things that immediately peg you as an outsider, like if the adult men in that area uniformly use shoulder bags or briefcases & you're carrying around a backpack.

It can also have a lot to do with nonverbal and body language–observe how close people stand to one another, how much eye contact if any, whether people greet strangers & if so how, how people queue, etc.. It took me a while when visiting India to learn that people don't stand on line the same as they do in the USA. I put my bag down in an airport line to get out a document, and as the line moved forward, people just started going around me, because I was not immediately moving forward with the line. In the USA this would be considered very rude, but in India (at least in my limited experience) it was considered typical–the line doesn't wait for you, move it or lose it. I got with the program quickly after I learned that.

And this should go without saying but don't display expensive or flashy things (jewelry, cameras, possibly fancy cell phone etc.) in an environment you're not familiar with.

Nowhere on earth will a nice button up shirt, dark sneakers or leather shoes, simple pants and a one colour sweater not be good