Turn Down Brightness
Smartphone screens are, with few exceptions, high-def stunners, trumping laptops and TVs in terms of pixel density (making video and images seem crisper). But nothing kills your battery quicker than that bright, beautiful screen. So drop your brightness whenever its feasible, which depends on what apps you're using, and where you are.
If you're skimming blogs in bed or in an otherwise unlit room, the bare minimum brightness will likely be fine. If you're outdoors in the sun, don't bother squinting at a darkened screen – use the quick-access settings to temporarily crank the brightness. But in general, you should be able to see the screen perfectly well at brightness levels far below what the automatic brightness feature selects.