My phone battery lasts often less than 3 hours. How do I get more life out of it?
FYI, it's a 3 year old Apple iPhone 5.
A few things I always do to try and preserve my battery life:
You can always check what is using the most battery if you go to Settings -> General -> Usage -> Battery Usage and then you can think about using that feature less or getting rid of it completely.
One thing to keep in mind is that the most power consuming thing is having the screen on, so having it off as soon as you stop using it and not checking it every 5 seconds will help big time.
Make sure that geolocation, bluetooth and wi-fi, 3G are turned off when not needed - they often consume much power
Also, when you don't use or don't need cellular communication, turn it off also (this and above can be turned off by enabling "Airplane mode")
Quit all the apps you are not using
Remove all the apps you don't need or turn off undeleteable default apps (Samsung option), because some apps have background processes which, of course, consume some power too
Set auto-turn-on and auto-turn-off when you don't need you phone, or just turn it off yourself and turn on again when you need it
Reduce the brightness of the screen
If your background on the main screen has a parallax effect, turn it off. Also applies to the transparency of notification center and control panel (which appears when you swipe from the bottom).
Don't allow your battery to become completely empty - it will worsen the life time of your battery
New Samsungs also have an option to reduce performance, fps, make the screen monochrome
In addition to the steps mentioned by @MrPhooky, following are regularly usable practices which won't restrict your frequent use of phone.
black background/wallpaper. Screens when black consume less battery.
keypad tones/vibration/haptic feedback. They are generally set to full volume from factory.
battery saving mode in location settings.
adaptive brightness, instead of a fixed brightness setting.
flight mode. That way you can use your phone offline & come back online with a tap whenever need to use network.
overcharging your phone; don't go to sleep plugging the charger. Also, contrary to @nicael, allow your phone to completely drain battery
once in a month or two. This helps.
It is a fact of life with current battery technologies that the useful lifetime of a battery is usually less than the useful lifetime of the device it was sold in. If you always buy the latest and greatest and replace your phone every year, you won't notice it. If you get second-hand phones, or change it once per decade, you're likely to need to replace the battery. Three years is a typical timeframe for a smartphone battery to show serious signs of wear.
On an iPhone, replacing the battery is difficult, but not impossible. (Why? Because covers that can open are considered ugly. That this incites you to buy a new phone for $600 rather than replace a battery for $30 is a side benefit.) For the iPhone 5 specifically, Apple has a replacement program, but that's only for a specific series where the battery suddenly gets worse. In general, Apple won't help you. iPhones are popular models, and you can find tutorials online — Ifixit is a good one. There's even a $25 specialized tool for that.
Alternatively, if you really don't want to open your phone, use an external battery pack. For $20–$100 and 100g–300g you can get an external battery pack that's about the same capacity as the internal one or several times that.