Q: I was stuck in traffic the other day with my smartphone battery down to 10%, but I was waiting for an important email. Is there anything I can do to save battery life when I'm not near a charger?
A: As a temporary quick fix, I usually switch my phone to 'airplane mode', which disables your device from transmitting signals (functions that use up battery life). When your phone is in airplane mode, you can't receive calls and texts, and the phone no longer sends/receives wifi or bluetooth signals. After all, if your phone shuts off at 0% battery, you'd be in the same boat, so you've nothing to lose.
Then, when you're in an area where you can check your phone, you'd switch off airplane mode which lets you quickly check your voicemails, texts, and emails, and then you can go back into airplane mode until your destination which hopefully has a charger. Another way of saving battery life is to dim your screen brightness; on Apple iPhones, this function is quickly reachable in the swipe up control panel, even in the locked mode.
If low battery life is an ongoing concern, you may want to keep your phone's wifi and bluetooth turned off, so that it's not constantly looking for wifi hotspots or bluetooth devices to pair with. You can set your phone to manually "pull in" your emails and notifications at the times when you want to check them, rather than having these items automatically "pushed" to your phone at timed intervals, since every time the phone checks for new emails, it's using up battery life.
Be sensitive to apps that use up battery life. GPS functions for location services, like Google Maps, will drain your battery. You can check the battery usage of each app on your phone. On iPhones, you'd go to "Settings-->General-->Usage-->Battery Usage" and a list like this will appear. For apps that run in the background, you can minimize their usage by going to "Settings-->General-->Background Refresh".
Finally, if you're a road warrior, extra hardware such as car chargers may be the long term solution. Android phones usually have swapable batteries, so carrying an extra charged lithium ion will help. Apple device batteries are hardwired, but there are chargeable phone cases, such as the Mophie power pack, that can give additional juice to your thirsty device.