I think one of the easiest way is using Android SDK tools. I'll explain this method using Windows, but in Linux or Mac should be similar. This is what you have to do:
-Connect your mobile to computer with usb cable
-Open your SDK folder.
-Go to tools folder
-Execute ddms.bat
-Your Device must be Online
-Go to Device>Screen capture... or press Ctrl-S
-Press Refresh in order to update your Screenshot.
-Press Save to save your Screenshot.
-Connect your mobile to computer with usb cable
-Open your SDK folder.
-Go to tools folder
-Execute ddms.bat
-Your Device must be Online
-Go to Device>Screen capture... or press Ctrl-S
-Press Refresh in order to update your Screenshot.
-Press Save to save your Screenshot.