This isn’t rocket science.
Use a protective case and a good screen protector. My favorite general use cases, at the moment, are those made by British Mous. They come in many different designs, are affordable and protects phones even in nasty drops. For screen protectors, I use different brands. The important thing is that it can withstand sharp-object-attacks and is easy to attach without leaving air bubbles between the display and the protective glass. I have used and been satisfied with amFilm and ESR protectors.
If you manage not to wreck your phone, the battery is the part that most likely makes it feel old. After a couple of years of daily use and charging, the battery won't be able to hold a charge for as long as when you bought it. Replacing the battery will give your phone new life at an often reasonable cost. Make sure to have the battery replaced by a firm with a good reputation. Some are less cautious or use cheaper parts, which can lead to trouble later.
Give it a little love. Don’t throw it on tables just because it should be able to handle it. Don’t put it in pockets where it doesn’t fit or may get bent (like jeans back-pockets). Don’t let it get wet unless it is water protected. Keep it at room temperature. Freezing temperatures will make the battery drain quickly, so keep it in an inner pocket rather than an outside one when the temperature goes below zero degrees. High temperatures (above 35 °C) can cause irreparable damage to the battery. Never leave your phone in the sun or in a car on a sunny day. It will quickly become warmer than 35 ℃.
If your smartphone gets drenched in water, and isn’t built for it, turn it off immediately. Keeping it on will most likely make it short-circuited and die. Instead, wipe it dry and put it in a closed plastic bag with dry rice for a week (yes, a whole week!). The rice will absorb any moist inside the phone. There are many stories of people who kept their wet phones in rice for a day or two and then had success turning them on. I strongly advise against this. If you are not that lucky, your impatience may instead ruin it.