#1: hit the home button twice, you will see the pages menu, get rid of all of them.
why does this work? when you have a page or game running even though you have the game or page closed it still uses battery in the background.
#2: don't have pokemon go installed, it takes up a ton of battery, about 3-4% every minute.
#3: swipe up from the bottom of the screen to the top, you will see a menu, turn the screen brightness down. (this method only works with iphones 5-7)
thanks, that's all!