My phone's battery was pretty bad (old note 5, retaining only 70% of battery life). It wouldn't even last me a day! It was also draining battery fast when the screen was off. Here's how I significantly improved my battery life:
0) Disable bloatware:
Go into your settings and find your applications. Then disable or uninstall everything you dont need. A lot of the time, system apps like Google Play Newsstand can't be uninstalled, but keep getting updated and stay in the background. Disable all of these apps. (Also consider disabling all the apps your carrier installed, since you'll never use it anyway). If your phone has a feature to keep apps asleep all the time, use it to keep several of your apps asleep (Samsung phones do this: Device Maintenance).
