I'm American, proudly so, cue the patriotic music. I can buy as many clothes as possible from Zara (believe me, I try), eat and drink like a local, even use some choice phrases in Spanish, but chances are I'll be recognized as an American when I'm out. And most of the time, I'm perfectly fine with that.
There are times though that I want to blend in a little more, especially if I'm in a crowded place or by myself, or both. Honestly, I've never had an issue while living here, never felt unsafe, and have only once been hassled for being an American (in Granada, and admittedly the woman was belligerently drunk). I guess part of my desire to blend in is that I don't like sticking out like a sore thumb. Safe or not, it can just be awkward.
Some things can't be helped. Graham's ginger status means that pretty much the only places he could be mistaken for a local are Ireland or Great Britain. I'm a little better off, with dark hair and eyes, which are very common in Andalucia, and being short (for an American, happily average here). Of course, those things only get me so far, the jig is up the moment I start talking.
Besides the obvious (ditching the American flag t-shirt or any t-shirt, really) there a few things that help when trying to blend in, like earlier today when I went to the Rota market to walk around. In the crowd, surrounded by cries of ¡todo a 7 euro! and ¡ole ole ole que barato vendo!, here are a few things to remember: