Although phone operators have become more generous when it comes to subscription traffic, it's a good idea to know how to save data on Android. 
It doesn't even matter that you have more and more data traffic included. In a context where there are too few opportunities for unlimited data traffic, it's very easy to end up being charged extra for watching too much on YouTube or Facebook. Even reliance on Google Maps as a navigation solution may cost you. There is no point in mentioning Netflix, SeeNow, Voyo, Dezeer, Zonga, Antena Play, Apple Music or HBO Go. All of these are black holes for your data subscription, which is why it's a good idea to consider a few methods of monitoring and managing data traffic on Android. If you have an iPhone, a few days ago I made a similar material for iOS devices.
How do you know what data traffic on Android you are consuming now?
Before any other precautionary measures, you must first know if you have a problem with the average data traffic generated per month. To be sure of the situation, either check the latest or the latest invoices or go to the web pages of Vodafone, Telekom, Digi or Orange. You create a free account on the operator's website and constantly check how much data traffic you've consumed. In addition, there are mobile applications from each operator for the same thing. 
If you don't have a data subscription yet and you don't know roughly how much traffic you need, a surprisingly accurate solution comes from American telephone operators. Go to the AT&T or Verizon website and start entering as accurately as possible the average number of emails you receive daily, how many web pages you access, how many videos you see on YouTube, how much you spend on social media like Facebook and not no more. You'll see estimated data traffic right away, and you can use that value to negotiate the subscription you need.
If you use a terminal with Android 4.0 or later, a very accurate statistic regarding mobile data traffic can be found in Settings - Wireless Networks - Data Usage. There you should see a window like the one in the first part of the screenshot above. A little below, on the same page, traffic will be broken down into apps, to help you better understand where your Android data traffic is going. It is very important to remember that all this information does not refer to WiFi, only to 3G / 4G.
From the same interface, using the two bars, orange and red, you can set a threshold from which you will receive a notification related to the traffic you still have available. The second bar, red, completely interrupts your data traffic until you intervene or until the subscription month changes. As an alternative to monitoring, you can use some third-party applications that we'll detail below. 
How to limit application data traffic over WiFi
When using a smartphone or tablet with internet access, you should keep in mind that in addition to manually generated traffic via streaming, Facebook, installed by applications or browsing the internet, there is also constant traffic generated in the background without no intervention from you. The latter varies significantly from application to application, but at the same time can be completely adjusted or disabled.
In principle, all restrictions that you will implement using the steps below will only apply to mobile data traffic. These restrictions never apply to WiFi. When you are near the wireless network in your home or office, everything will be normal. 
Significant data consumption can be derived from updates to applications that you already have installed. Although these updates need to be installed regularly, it would be preferable to force your smartphone to download them exclusively over WiFi. To do this, go to the Google Play Store, press the on-screen menu button, go to Settings, and check Update over Wi-Fi Only.
To see which apps are responsible for a significant amount of background downloads, go to Settings - Wireless & Network - Data Usage and tap any of the apps in the list. You'll immediately see images like the ones above and a distinction between traffic when the app is in the foreground or in the background.
The simplest option, when you don't know who is generating your data traffic in the back, would be to tick, simply and quickly, Restrict Background data. The problem with this solution is that you may not receive new emails unless you enter Mail, as well as WhatsApp messages or other chat and social networking applications. As a result, it would be best to go to the traffic details page of each application in the Data Usage menu and check Restrict Background Data. 
Some applications also have an internal menu to restrict the traffic generated, but this is not a general rule. For Facebook, for example, if you limit the number of notifications displayed, you'll gain both data traffic and battery. The difference is that when you choose to reduce the traffic generated inside an application, the restrictions will remain in force on both 3G and WiFi.
Android data traffic management through third-party applications
If the solution proposed by Google in Android does not seem to you to be sufficiently well developed or is too complex from your point of view, there are a significant number of alternatives that you can use. Illustrated in the screenshots above is Onavo Count, which can be downloaded for free from Google Play. It works on any version of Android later than 2.2 and gives you access to as many options as you want.
Basically, you can even call widgets on the home screen to make sure you don't miss anything. The first time you run it, you'll define the date your subscription changes and the included data traffic. After that, it will take on a significant number of management tasks. With very conclusive notifications, Onavo Count - Data Usage tells you in advance if there is a potential problem. It may also warn you that you are experiencing internet traffic problems based on a prediction based on your current habits.
If you want a quick and easy way to stop mobile data traffic in a flash without going into Settings, a utility like Data On-OFF, free from Google Play, fixes the situation. It would be ideal not to resort to this path too often, but in the absence of time, it is ideal.
The secret is in the small details
If you are used to using a browser such as Opera with real-time data compression, you will gain a lot of traffic from the start. Google Chrome also includes compression settings, but they're not enabled by default, and you'll need to scroll through the settings in the program to get them up and running.
The ads you see in your favorite apps are also responsible for a lot of traffic. The solution is to buy for a fee the applications in which they appear, usually at a price of one euro. Programs like Adblock Plus or AdAway do an almost as good job of removing ads. Unfortunately, the latter need root rights on the phone to block ads displayed on 3G. By default it only works on WiFi.
Last but not least, one solution is to start using applications that support saving data offline. With a navigation app like iGO, Navigon, or even Google Maps, to some extent, it's best to have maps saved on your phone before you go. Streaming applications such as Zonga, Deezer or Spotify also support saving maps.