In one sentence, describe what the following article is about:

Turn off GPS, auto sync, notifications, updates, and wi fi. The only thing you need is your phone's network signal. If you use an Android device, go to Settings Apps Running and close all processes. Go to All Apps and disable any Apps you don't need, such as Hangouts or Play. If you use a Windows Phone 8.1, turn on Battery Saver. Put your phone's brightness on minimum. If possible, remove your phone's SD card. This can be a significant battery drain on some phones. Keep sessions short. If you'd like a longer internet session, limit yourself to emails and simple web pages. Avoid streaming videos, downloading, and browsing in multiple windows. Check your phone settings to see if there's a tethering range option. Lower this to the minimum range and keep your phone next to your laptop. Most modern phones can be charged by plugging them into the USB port of your computer, although you may need to buy the right cable. Note that charging from your computer is much slower than charging from the power outlet on your wall. You may even be able to tether to your computer via USB at the same time as charging. This may be useful if your phone won't charge from your laptop, or if your laptop has a short battery life. This is also sold as a "power bank." Some companies, including EE in the UK, sometimes offer these for free. Be sure to check your service provider's website before purchasing one. If your phone's battery is removable, this will get you double the internet time. Remember to charge the backup battery as well once you get home.
Turn off all unnecessary phone tasks. Turn down brightness. Remove the SD card. Stick to lightweight internet use. Lower your phone's tethering range. Plug your phone in to your laptop. Buy a portable phone charger. Carry a backup battery.