Data? Don't talk to me about data. Mobile 4G data is the most expensive part of my smartphone habit. One recent morning, as I browsed Twitter in a coffee shop, everything stopped suddenly because I had already burned through the month's data allowance.
You've probably had similar experiences. Apps that monitor data usage and alert you when you have used too much could be a good solution.
For iOS users, the simplest and clearest app is DataMan Next by Johnny Ixe. It features clear, large text that allows the app to tell you much of the information you seek in one glance.
Once it is set up, the app runs in the background, gathering information on how much data you are using over a mobile data connection such as 4G or over Wi-Fi. Opening the app brings up the home page, which has a big percentage number in the middle, a counter to when the billing cycle ends and simple displays of how much you have already used on cellular or Wi-Fi. It also looks at your use patterns and predicts whether you are "safe" for the month's limit. The home page color changes from green to red as you approach the limit.
The app can also send alerts as you hit different percentages of your data allowance. The app costs $2.
My Data Manager is an alternative, available on both iOS and Android. It is a bit more fussy. It is free, though, and provides a more thorough measure of data habits. It has a quick-glance home page to report the data used in a month and how much was mobile versus Wi-Fi. It even has a section to monitor data use when you are roaming overseas.
If you tap on the small graph icon on the bottom of the screen, the app will display a bar chart of your data use on a daily basis. You can also see a map showing where you downloaded data the most. The idea is to give you insight into your data habits so that you can modify them sensibly.
Another good option is Onavo Count, free on Android and iOS. The Android version looks well designed, with a minimalist interface and intuitive graphs. Its home screen tells you how much of your monthly or weekly data you have consumed with a display of shading on a colored ring.
On the home page, it shows how your data use breaks down into categories such as video-viewing, use of social media and so on. It can also show how your use of particular apps compares with that of the average user.
Other possibilities are the free Data Monitor by Nutec Apps on iOS and 3G Defender on Android. They work similarly, but look more cluttered and feel different from the other apps mentioned here. It is also worth noting that in iOS 7 and Android 4.x, Apple and Google have included systems to see how much mobile data is being used. But you have to dig into the settings to find them, and the information is not necessarily as complete as that offered by the apps mentioned here.