Being a tourist is always a very good thing. To be the personification of the stereotype of a tourist is not very good. Avoid carry the tour guide book in hand, wear socks with sandals and please leave the selfie stick at home. These are the first steps to avoid loolink like a tourist.
The best thing about traveling is being able to live a different culture so the key to do that is to blend in with the locals, not stand out. This way you can get to know the reality of new places and new people more closely. Also, by acting as a local you have less chances of being mugged or tricked on your next trip.
How to NOT look like a tourist in your trip
Avoid showing large amounts of money in public
Pay attention to the way you expose and carry money during your vacation. Avoid showing large amounts in a crowded location. Find out how the ATM usage fees work before you travel to a foreign country. Sometimes is better to use your ATM card and get small amounts of money at a time. Know also the currency of the country to avoid embarrassment when paying your bills.
Adopt the local dress code
Wearing the clothes from local souvenir shops will not make you go unnoticed for sure. We know that you love porto, however, it is not necessary to express it on a I (heart) Porto T-shirt. Check the weather conditions of your destination and dress accordingly. Pay special attention to how you dress if you visit religious monuments. Remember that it is very important to respect different cultures.
Adopt the local habits and customs
Do some research on the social behavior of places you plane on visiting. It is important to understand the habits and customs to adopt a correct posture. Certain behaviors of your culture may be offensive to other cultures. Again, notice the basics of the habits and adopt them during your stay. Blend in, don't stand out.
Learn the local language
At least learn how to speak the universal language: English. And if you have the time and patience, try to learn a bit of the language of your travel destination. The effort will be valued during your stay and in addition, knowing another language is always an advantage to you. Be very careful with incorrect translations! In two distinct places of the country the same word or expression can have different meanings, even within the the same language.