I have a smartphone thats over 5 years old, and battery still lasts whole day. I charge it just once a day, and I use it all day.
Its not even an expensive smartphone or anything special.
I simply followed 2 basic rules:
1. Dont charge over 80%. Dont let it drop below 15%.
It may seem like you are going to get more battery by charging to 100%, but this actually makes battery last much shorter.
So your "100%" will just turn to "40%" after 2 years, because charging to 100% damages battery over time. Charging to 100% only works in very short term.
2. Reduce screen brightness to lowest
Screen brightness is the main drain on battery. By reducing screen brightness to lowest, battery should last at least twice as much.
Now, power saving option can reduce brightness even further, which gives you even more battery life.
Reducing brightness has 2 effects. First effect is that battery lasts for entire day, so you dont have to charge as often. Second effect is that battery itself is healthier and will remain good for years because of reduced number of chargings.