There's that point around three o'clock in any office, when the call goes out. "Does anyone have a Samsung charger?" says your oblivious colleague, hunting round for a microUSB cable for his poor dying phone. But you don't have to let it come to that. Although smartphone battery life kinda sucks, there's a whole host of things you can do to make it last longer.
The display on your phone is typically responsible for around 50 per cent of battery drain. Some will advocate turning down the brightness until you need immersion-tank levels of ambient light to read it, but to save both your peepers and battery life, trying turning on auto-brightness. A default feature on every single non-completely-budget phone nowadays, auto-brightness uses the ambient light sensor in the phone to set the screen backlight at just the right level, so your screen's readable but not wasteful. Can be found under Display settings in most Android phones, or under Brightness for iOS.
Another way to save on screen power is to bring down amount of time the screen stays active without any inputs. Typically, the default time is around two minutes -- which is way too long. Honestly, if you're staring, immobile, at your phone for 120 seconds straight, you've probably got bigger issues than battery life. Again, the setting can normally be found under Display for Android, or General-->Auto-Lock for iPhone.
This one can definitely be filed under 'marginal gains', but the colour of your background image can have a small effect on battery life. We're talking in the region of one or two percent here or there, but a white background taxes the backlight more, using a bit more power over a dark or black background.
All push notifications (that is, notifications which have a constant connection to the server, and thus are 'pushed' to your phone instantaneously) tax your battery life, but probably none more so than email. Under Mail settings (the setting is in the general settings for each account on Android, or Mail-->Fetch New Data on iOS) change your settings to Fetch, and then five or ten minutes will probably do it. If you've got a couple of email accounts, and some are more important than others, you can set any spam accounts to Manual. Honestly, you don't actually need to know about the 50 per cent off at Domino's.
Two of the worst unnecessary battery drains are Bluetooth and Wi-Fi. Often, people leave them running in the background simply because they don't realise they're on. Having Wi-Fi and Bluetooth futilely hunting for signal all day will probably cost you about 15 per cent of your battery. The one exception to this is don't turn your Wi-Fi off if you've got a connection to an actual hotspot. Using data over Wi-Fi is more battery-efficient than using cellular data (not to mention it's better for your data cap, too).
While we're on the matter of unnescessary radios, let's talk about GPS. There's a whole bunch of apps that use GPS, that probably don't need to. Do you really want location-based iAds, or Twitter to "optimise your experience based on your location?". Didn't think so. The fact is, apps other than Weather and Maps use GPS all the time, and each time your phone has to hunt for satellites savages the battery. Using the Location Services settings, or in-app settings for that matter, you can select which apps should be allowed access to your location.
This is a bit of a DEFCON5-red-alert-ohmigod-only-9-percent-left measure, but disabling high-speed cellular internet can help massively to eke out an extra few hours' usage. The 3G and 4G radios use more power than their 2G or EDGE counterparts, so turning them off can help if you really need a bit extra. The option's available under Mobile Networks on Android, or Cellular on iOS.
You can see our full article on correct charging here, but the executive summary is: try and keep the battery north of 50 per cent; don't let it run all the way down; keep the handset itself cool, and avoid wireless charging.
Multitasking is great and all, but sometimes, you’ll have a bunch of apps running unnecessarily in the background, chewing through your battery life like a dog through brand-new shoes. On Android, you should go through closing background processes once in a while (just swipe apps sideways from the multitasking menu); on iOS 7, you can go General-->Background App Refresh to selectively turn off what apps will use data in the background.
Although all the tips above are true, their relevance depends on how you use your phone. The best way to get a grip of the battery life situation is by knowing what apps use the most battery. On Android, this is pretty simple -- just go to Battery, and you get a cool breakdown of what uses the most power.
For iOS, you're best off downloading a custom app. Battery Doctor and Battery HD+ are both pretty good for this purpose, giving you a load of information on what uses the most power, and also some data on how best to charge your particular handset.