I keep a charging cord in the car, and have a couple of small portable backup chargers that I occasionally carry, but I've taken a few additional measures on my new battery challenged phone:
1) I have an app called Brightness Level, and find 50% is a comfortable setting. Your phone will tell you that your display usage is the single biggest energy hog.
2) My new phone has a Power Saver mode. Not sure what it does yet, but I've turned it on, and am not seeing any loss of function.
3) I've switched my app update settings to "WiFi Only", which means I won't be dragging down my battery with app updates when I'm out and about.
