It depends a lot on where you are going. When I go to Japan I cannot blend in no matter what, even though I speak the language. If I want to look "less American," I speak a lot of German or Spanish or perhaps wear a Maple Leaf tee shirt.

Seriously though, I don't think the trick is to look less American but rather to look less like a tourist. I don't wear Bermuda shorts, sandals, Hawaiian shirts, camera on a strap around my neck or anything else typically "tourist" and let the rest of the chips fall where they may.