Packing a backpack for a journey of a week or of a year is more or less the same process with the same challenges: you need to pack useful and light. Don’t forget that travelling with a backpack is more or less like travelling with your house on your back. So you want it to contain all you need but without breaking your back!
Here are some tips on what to pack.
You want to make sure you have some practical, easy to wash and dry, adaptable clothes so that you don’t need to bring a lot. In most of the countries you will visit you will probably have some good shopping options to replace or complement your assortment. Actually you may even want to keep some space to buy some cool local products!
Therefore at minima you should leave with 2 T-shirt, 1 short, 1 trousers, a set of 2 underwear and 2 anti-perspiration pair of socks. You may also want to bring a sweater even if you are going in warm destinations because even there AC can give you chills.
Also I would recommend bringing a waterproof/ windproof jacket for those days of pouring rain and something to cover your head for those days of strong heat.
Finally for girls you should be a polyvalent large scarf (pareo/ sarong) that can help protect you from the cold, the sun and even more important that you can use to cover-up when visiting religious sites.
A good pair of walking shoes and a pair of flip-flop is the bare minimum you will want to bring to almost all destinations.
Flip-flops are super useful in many situations: beach, shared bathroom, indoor shoe…
As for the walking shoes they need to be light, solid, adapted to different environment and if possible waterproof (Gore-Tex shoes are usually a good choice). A good example of this kind of shoes is the trail shoes. Unlike the flip-flops that are usually easily replaced, you will want to take good care of your walking shoes.
In addition, depending on your needs and your destination you might want to add a third pair of shoes: a pair of (comfortable) sandals or sneakers for every day (for instance in urban areas) or even an actual pair of good hiking shoes if you plan on hiking a lot.
You can find all the basics in almost every country so no need to have a full stock upon departure!
But make sure you still leave with a toothbrush, toothpaste, deodorant, shampoo/ shower gel for the first days. Be careful to have good closing caps (you don’t want your shampoo to open and spill everywhere!).
You can also try to choose products that are respectful of the environment as to limit your ecological imprint in the countries you visit (bamboo toothbrush, natural and organic care products…)
Don’t forget that you will be on holidays so probably quickly tanned (don’t forget to protect yourself properly!) and relaxed so you will most probably be naturally beautiful! If you still feel the need to have some basic make-up on, keep it to a minimum and practical size. Don’t forget that some make-up textures might not stand being exposed to heat. My go-to is usually crayons; but powder would be ok too. Avoid sticks and waxy texture.
Pay a visit to a travel doctor before you go who will give you a prescription for a complete first-aid kit. Plan a visit ahead of your departure to make sure you are up to date with your vaccines: if not you may need a couple months to receive all the necessary shots!
Also make sure to have a good sunscreen (minimum SPF30) and some mosquito repellent. If you are travelling during rainy season you may also want to bring a mosquito net.
Again you want to travel useful and light so you need to limit yourself (and it is not easy if you are not used to it!). Here are some accessories that can come handy. You probably won’t need more than this. I am excluding the case here of going camping!
Sunglasses: do I really need to explain why?
A discreet pocket: sometimes you need to carry with you your passport and some money for more safety. But it is only safer if you can actually carry them in a discreet manner this is why you may want to invest in one of those flat pockets you can wear under your clothes. You now also have new options on the market: a pocket hidden in your scarf, one to hide in your bra, in your belt… you can DIY easily most of these options.
A day-bag: you don’t want to bring your big backpack all around if you can leave it in your hotel! So you may want to bring along a small day-bag to carry only the things you need for the day. Shoulder bags can be handy and relatively safe but they can also harm your bag if they get heavy. The best is a small backpack but beware of pickpockets!
A waterproof bag: you can get a sealed bag or even just good Ziploc bags (the best have 3 closing rows). It can come really handy to protect your papers and electronics under the rain or even at the beach (sand is also your electronics enemy!)
Microfiber towels: not everywhere will you have towels available so make sure to bring one with you. Microfiber towels are the best option as they are light and dry fast!
Sleeping bag (liner): if you are traveling on a budget you may end up sleeping in places where the bed linens cleanliness is questionable. This is where the sleeping bag is more than welcome! Silk ones are a bit more pricey but very light and comfy. You can also find them in many of the countries used to backpackers (especially SEA)
Light: Power-cut or dark alleys, there are many situations where you will be happy to have a light. Dynamo light, head light or torch it is your call! Just make sure it is light and the battery stays long.
Tweezers: out of the cosmetics aspect of it this can be very helpful to take out a splinter of a sting
Lockers: Not only do I always recommend to lock your bag if you can, you may also need to lock your storage box (some hostels charge you a fee to rent a lock) or even a cupboard in your hotel room for more safety.
Swiss Army Knife: I do agree that it can come very handy but remember to always put it in your checked-in bag if you don’t want to have it confiscated in the airport. It also means that if you bring one you will need to check-in your bag.
Passport cover: cheap and simple you may want to protect your passport as this is the most important document of your travel! Make sure to scan it and have a copy with you too!
This part is really depending on your tech and photography attitude. Therefore please read the specific article for it.
At minima I recommend however to carry at least a basic phone with your emergency contacts registered and an international SIM card. You can buy local SIM cards if you need.