There are many ways to backup an Android phone, but if you were to ask me about the best way for a rooted Android device, without any second thoughts I would suggest the Nandroid backup.
Nandroid backup is not like other backups that create different backup files for distinct settings. It creates a mirror image of the current state of your ROM to an image file that includes everything and when you restore it, every tiny detail (like call records and widgets) will be restored to your phone.
Step 1: Boot into recovery mode and after the ClockworkMod recovery is loaded, open backup and restore from the list of options provided on the main menu.
Step 2: Again select backup from the Nandroid backup sub-menu to start the backup process.
Step 3: The recovery will then calculate the free space available on your device and start the process if sufficient space is available.
The process may take some time depending upon the size of your ROM. After the backup is successfully created, you will be taken back to the CWM home screen.
Restoring Nandroid Backup
When you wish to restore the Nandroid backup, just select the restore option from the Nandroid sub-menu in the recovery. If there’s more than one Nandroid backup saved on your device, the phone will prompt you about the multiple image files present, and that you can select one of them to restore.
That’s all, Nandroid backup is one of the most easiest and effective backup mechanisms one can get for a rooted Android device. Always remember to take a Nandroid backup before you consider doing some major changes on your Android.