Not saying the magic words
People will know you're a tourist, so don't worry about blending in. Instead, focus your energy on memorizing three simple phrases: "please," "thank you," and "excuse me." When you've got manners, the world is your oyster.
And don't assume that everyone speaks English. A lot of people do, but they'll appreciate you trying in even the most basic way to meet them on their own terms. In cities like Barcelona, which uses Catalan instead of Spanish, a few words can make a big impression by demonstrating your understanding of regional differences. Many people who understand English in tourist-friendly cities will see you coming and will switch automatically.