1) Make an effort to learn at least a few words in the local language. "Hello" (and its various translations) being the most important. It isn't difficult to learn one word, and it makes a big difference. Do not expect everybody to speak English, unless it is an anglophone country. Even then, it is not a given. Languages are for communication, so don't worry if your language skills are not perfect. Just. Make. An. Effort.
2) Do your research. Read about the scams you might encounter, the reputation of the city and its people, the insider tips on what to/what not to do. When you arrive, don't be daft, don't be naive, and have a little bit of cynicism. Do not accept items from strangers, especially if they are "free" or oddly cheap.
3) Be aware of cultural differences - your version of politeness is not necessarily everybody's version. In some countries, queuing is not a thing, offering/asking for help can be seen as suspicious, and wearing shorts and a singlet in a church is offensive. Never say "this would never happen in my country!" - you are not in your country. No one cares.
4) If you are going to a popular tourist destination (e. g. Paris, Rome), expect it to be a bit smelly and dirty. Where there are a lot of people, there will be damage caused by said people - locals and tourists alike. There may also be a lot of beggars and homeless people, as these cities often have a high cost of living. Give if you like, or even stop for a chat if it is safe to do so, but otherwise avoid eye contact.
5) Be aware of the foreign stereotypes of your own country - especially if you are American. Ask a citizen of any other country in the world about American tourists, and you are sure to get a lot of complaints about how loud and demanding they are. If you are like that, make an effort to tone it down. If not, continue being a good ambadassor for the US! Other countries have some negative stereotypes, too. Observe how the (respectable) locals act, and make an effort to fit in with that.
6) Don't get in the way. For example:
- if you are walking down the street, be aware of other people around you also walking, passing, going in different directions.
- when taking public transport, do NOT stop in the middle of the path. If you are lost, move over to the side.
- if you are on an escalator in Paris (and some other cities), move to the right like most other people. The right-hand side is the slow track, and the left-hand side is for those in a hurry.
7) In some countries, the customer is NOT always right. If you receive good customer service, let someone know by leaving a review, leaving a tip, or simply complimenting the staff member/management. If you receive bad customer service, let someone know by leaving a review, NOT leaving a tip (unless you are a country where tipping is necessary), or discreetly informing management. Do NOT make a scene.
8) If you are lost or in trouble, keep calm, ask for help, contact your local embassy/overseas travel service/travel agent/etc. if necessary, make use of the internet, contact family/friends. Make a plan beforehand of how to approach emergency situations. Of course, one can't plan for everything, but if you inform yourself ahead of time about available services from your own country or in your destination country, you will be in a better position to resolve the situation. Take out travel insurance, just in case. Scan copies of your documents and keep them somewhere where you can easily access them - email them to yourself, upload them to the cloud, take screenshots, etc.
9) Make a plan of things you want to do/see/eat/etc. during your time away, but keep it flexible. Allow for plenty of time between activities. Allow yourself at least an hour more than you think for transport if you need to get to airports/ports/etc. Download tourist apps. Look into skip-the-line tickets and tours (but be aware that not all are legitimate). Read reviews.
10) Be a decent human being. Respect other people, but also be aware that there may be some unsavoury characters that you will meet along the way. Your own/your friend's experiences are not necessarily a true reflection of the city and its people. Remember that your destination is not your home; you are a guest, and you should be the sort of guest you would like to invite back to your own country.