Since pretty much the dawn of the Industrial Age, shoes have been made like this: pieces of leather and other materials cut and sewn together in what's called the "upper," then glued to a hard sole. Despite the machine element, much is still done by hand, with skilled workers needed to put the shoes together; materials are often sourced from different places, with shoes shipped back and forth several times before completion.
The shoes upper is by far the most important part of the shoe. Once this is gone, the shoe is no longer salvageable. Think of the leather on the upper as your own skin, only that it is no longer alive and does not have a body to take care of it naturally. You, the owner, replace that ever-working body, and are thus responsible for the maintenance and upkeep of the skin. Now there are many ways to do this. Naturally, you will want to think of the things that your leather does not like, for example water, wine, beer (especially — I know!!) or any liquid for that matter, dirt, food etc. You need to make sure that you can hopefully limit the amount of times that your upper leather has to make an encounter with these things, because the less able you are to do that, the less likely that your upper leather will last long. Now obviously you go through life and do many things and it makes no sense to tip toe around on your nice shoes hoping to never ruin them, but there are precautions that you can take, such as checking the weather and having galoshes to be prepared, not wearing them to the pub when you know that you are going to get drunk or always using the sidewalk (pavement) instead of walking on grass or dirt etc. These are all common sense type things that I believe many people forget to think about but then are so mystified when they ruin their shoes quickly.
Now outside of being observant to your surroundings, there are several things that you need to maintain your leather. Those things are: a good leather conditioner (like the one that I sell), some good wax polish, a nice and soft horse hair brush, shoetrees, dust bags, some nylon (women’s tights) and ideally the box that the shoes came in. With the proper use of these things (and taking into account the paragraph above), the life of your shoes could increase ten fold. The most important things, in my opinion, out of that entire list, would be the conditioner and the shoetrees. Both of them act as deterrents to things that destroy your leather e.g. liquid of any kind (even sweat) and drying out your leather. So, if you can’t have them all, at least have these two.