Donβt be an arrogant jerk. You are a guest. Be modest, donβt assume you are better or wiser than the locals. Donβt criticize things you know nothing about.
Don't get upset when someone is not fluent in your language. Speak slowly so that others have a chance to understand you. Try to learn some of the local language, don't just expect everyone to know yours.
Don't complain about "police corruption" if you get caught breaking the law.
Most people don't like to have their photos taken by strangers. Would you like to have tourists taking photos of you or your grandma without permission?
"Slum tours", "child photography": do you walk around your hometown and take photos of children and homeless people? Why do it in India or Kenya?
Don't do damage to ancient sites. Do not take "souvenirs" (pebbles, rocks). Do not touch sculptures or paintings.
Don't litter, even if you see others do it. Minimize your use of plastic. Many countries lack the facilities to recycle or dispose plastic so it ends up in rivers and oceans.
SPENDING MONEY
In developing countries, support the local economy. Avoid foreign-owned hotels and restaurants. Buy locally produced goods. Support traditional skills. Give less popular establishments a chance. Don't just spend money at places that got many online reviews or a mention in your guidebook.
Luxury uses up a lot of resources. This comes with negative effects: 1. destruction of the environment (logging, mining, pollution, waste); 2. Local people can no longer afford some of these resources (e.g. housing, clean water, electricity). Sometimes indigenous people are displaced to make space for your resort. You are giving money to a very rich class in the tourism business, not to ordinary people.
Human/animal exploitation, drugs, products made from endangered species, stolen artifacts: don't create a demand for them.
Giving money to scammers, corrupt officials, dishonest sellers makes the problem worse. These people are very rich by local standards and don't need your generosity.
Bargaining is expected in many parts of the world. Don't overpay, but also donβt bargain too far. The seller may be desperate to earn some money, but paying 4$ for handicraft that took a whole day to make is disgraceful.
Traffic and pollution are making many cities unlivable. By driving a private car or taxi you become part of the problem.
Most modes of transport run on fossil fuels. Cars, planes, and cruise ships are the worst (per distance per passenger). The more crowded your transport, the less you pollute. If you drive a car, share it with other people. Try to replace frequent travel with infrequent long-term travel.
It can be difficult to say no to begging children or disfigured people, seeing that some of them really need help. Getting a job is unrealistic for many, especially the homeless and the disabled. However, there are many reasons against giving them money: it encourages them to embrace begging as a way of life instead of seeking a way out, it discourages parents from sending their kids to school, it motivates them to harass every passer-by, they may be exploited by criminals, they may spend the money on drugs, they may not be poor at all...
Similar arguments are sometimes made against welfare benefits (e.g. unemployment or disability pensions), but these are very different. Welfare allows people to invest in themselves and find a better job eventually. Unfortunately, since begging takes as much time as a full-time job, in most cases a successful beggar will not be motivated to seek a better job or to get an education - begging is the job. If your disability helps you win sympathy and earn money, you will be motivated to aggravate and expose it instead of treating it.
On the other hand, a pension that someone in a rich country receives could sustain perhaps a hundred people in a poor country. If you live in a rich country and support the welfare system, you have no excuse to be against giving aid to people in poor countries.
NGO/government programs that are supposed to help the poor are inadequate and often corrupt or come with a political/religious agenda. Many people remain without any help at all. For some, begging is really the only way to stay alive. So even though giving money to beggars creates problems, it is probably better than not helping at all, and can, sometimes, save a life. However, donating to a reputable, secular charity that fights disease and poverty is much safer. It doesn't make people waste their time begging in the street and can eliminate the cause of poverty for some people. The fact that you don't see them personally thanking you doesn't make it less valuable. Besides, as a foreign traveler it is difficult to make the right judgement about who really needs your help and who doesn't. A charity evaluator (e.g. GiveWell) can help find cost-effective charities.
If you must, better give food instead of money and not to people who harass you. Even better: buy something from street vendors, stay at a family-run guesthouse, get a ride in a rickshaw instead of Uber, leave a tip for good service.
Even when you reject a beggar, don't be rude about it. They are likely having a much harder day than you so there's no need to vent your frustrations on them.
In rural areas you may get surrounded by a bunch of kids asking for money or sweets. Most likely they're not real beggars but are playing a kind of game where they occasionally get rewarded. Be nice to them, but don't encourage begging (and please, don't take selfies). You can do much better by buying something from adults. In a big city, be careful: the kids may be pickpockets.
VOLUNTEERING
Be skeptical about any volunteering program that costs money to participate. If you have to pay in order to work, then most likely your work has little or no value and only your money matters. Sometimes it may be a scheme to exploit the people you are "helping" (e.g. "orphanage tourism").
If the work you do is profiting someone then you are denying work opportunities to local unemployed people who can't compete with free labour.
If you genuinely want to help, you can do a lot of good if you are skilled in a field that lacks funding in poor countries (e.g. health professional, scientist), or as a human rights/environmental journalist. If you want to volunteer as an unskilled language teacher, go to a remote/impoverished area and look for schools that don't have language teachers.
Missionaries: How would you feel about having foreign missionaries coming to your hometown, teaching children and poor people their foreign religion and family values, promising redemption and eternal life?