When your phone cannot use WiFi, using data traffic may be the choice. Here I have some tips for saving your data roaming.

Tip 1: Close the auto-sync
You can set it through settings

Tip 2: Turn off the Mobile network when you don’ t need network.

Tip 3: Close the apps that running on the backstage. Get some App that can do it. Usually I use Task Manager.

Tip 4: Try to download Apps from your PC not your device. Of course, it is a exception if you have enough data traffic.
For me, I always download APK file from PC, then I use a PC Manager to install the apps to my device. En, I am using Moborobo. You may also use AirDroid. Both are OK.