It you have a chronic problem with your cell phone's battery running out on you before the day is over, there are some things you may not be doing to preserve battery life.
Stop or limit the use of these power-eating features.
4G improves the speed of downloading large files. If 95% of what you do is email, texting, and Facebook, 4G doesn't improve your experience enough to bother with. You can always switch it on when needed. It consumes a lot more energy. When you are out of range of 4G, if 4G is on, it can waste a lot of battery strength seeking 4G.
Much of what I said about 4G can be said about wi-fi. Wi-fi is a blessing when your cell signal is very weak. However, it uses more power and, as with 4G, it can waste power seeking wi-fi. Switch it off if you don't need it.
Consider switching off GPS, but since GPS has some safety applications, this is a difficult decision.
I used to leave vibrate on always. Now I only use it when I need it. Anytime you are using your battery to drive motion—and vibration requires motion—you are using up energy. Make sure just to turn it on when needed.
Switch off any location-based apps. Foursquare, Fandango, Goggles, and many other apps use GPS and are always polling it to keep track of where you are. Turn off as many of these services as possible. You can always turn them on when needed.
Turn off as many notifications as you can. Some people are addicted to Facebook and want to know the instant something happens. Me? I can check once a day and that's enough for me.
Dim the screen.
Use screen lock to avoid pocket/butt dials, if those are happening fairly frequently.
If all else fails, get a spare battery or an external battery pack. A case with a built-in external battery is available for some phones.
Batteries become less efficient over time. If your phone's battery life had been degrading, get a new battery.
Use airplane mode even when not on a plane.
Keep your phone cool. Hot batteries drain quicker and frequent exposure to heat reduces battery life.