This is the most common one, but reiterating – Travel light. You won’t be using a lot of stuff you think you need. It is extremely useful in cases where you have to walk carrying your luggage, which is not an uncommon occurrence. I remember running a km with my 9kg backpack to catch a train in Oslo. I might have missed the train if it was heavier. If possible, keep it below 7 kgs as many low cost airlines allow that much as cabin baggage so you save on checked-in luggage.
Planning is good, but try to keep your plans flexible. You don’t know which place you want to spend an extra day, or which place you want to leave early. Sometimes plans change due to unavoidable reasons and booking in advance turns out to be more expensive. Not to mention last minute deals in hotels.
If your plans are flexible and you need a return/onward ticket for on-arrival visa, use https://flyonward.com. Faking a ticket might work as well but can be risky. Use https://www.booking.com for booking as most hotels can be cancelled for free.
If you are traveling solo, prefer hostels over hotels. You get to meet other travellers and some of them will turn out to be absolutely amazing.
Buy a travel sim if you are visiting many countries (e.g. Europe), or local sim in each country. Save data by using free wifi as much as possible. Most cafes around the world have free wifi. Always keep an offline copy of the current city in google maps for cases where you can’t access internet.
Whenever you reach a new city, get hold of a tourist map along with information about public transport, city centre, important places to visit, etc.
Be safe. Don’t wander around shady areas late at night, especially if you are traveling alone. Learn about safe and unsafe places from your hotel/hostel reception guy.
Buy comfortable and weather resistant shoes and jackets if going to cold/wet regions. One pair of shoes and one pair of slippers are enough so don’t carry more.
If you are from a developed country and visiting a developing country, always drink packaged water and be careful with street food. Otherwise or once you get a hang of it, always eat street food 🙂
Stay close to the city centre. Public transport is always available and things are easily accessible.