Most of my wisdom in this topic has come at the unfortunate hands of having done things the wrong way and that is why I am so adamant about people not repeating my mistakes. These are some critical ones that many other veteran backpackers can identify with years later because most of them made the same mistakes too.
1) Bring sunscreen from your home country
This isn't applicable to every country that you might visit, but in the time I spent particularly in South East Asia I found that sun cream costs an absolute fortune in these countries.
This is especially important if you are terribly partial to one particular brand as it has been my experience that the shops / pharmacies simply have a mish-mash of whatever in stock and a lot of it isn't waterproof. It was always around 2-3 times the price of what it would cost back home though, and that is heartbreaking. If you see a knock off brand while traveling, it is probably better to just ignore it because most of the time (in my experience) it was garbage and that is why it was so cheap.
Why is it like this?
I can't say for certain, but I think it has a lot to do with the fact that the natives of these countries are very rarely interested in spending lots of time in the sun the way that we do in the west. The idea of sunbathing is absurd for a vast majority of the locals and they think it is crazy that we actually choose to do so.
2) Bring a Nalgene or similar container
As the areas that backpacker frequent become more and more environmentally aware there is a genuine movement to provide water filling stations many times for free. Backpackers tend to visit poorer countries for the sake of saving money and unfortunately these places also tend to have pretty horrible waste management.
While this wasn't necessarily true when I started backpacking in the late 90's and early 2000's, it is pretty universal that guesthouses and even vending machines have filtered water for sale or even for free in some places. Not only are you saving money here, but you are doing a great service for the area you are visiting. If you don't feel like hauling around an indestructible nalgene, at least look out for these station with a water bottle of your own and refill it.
In many of these communities, putting something in the trash in no way means that it is going to be taken somewhere and disposed of properly. Every little bit helps! Right?
Mostly I tell people to bring as little stuff as possible on their trips, but i sincerely feel that a few items are truly necessary and these two are some big ones in my opinion. As always I will remind people to bring as few clothes as possible, especially the cotton kind. All of that stuff is cheaper in the countries you are going to be visiting anyway.