After install and configure many things on my Raspbery Pi, it will be too much if I have to do it again. Backing it up is really a good idea.
I need A SD card reader and a computer with Linux on it. Insert the card into the card reader, plug it into the computer. Find out where the id of the SD card using this command:
Mine is /dev/sdb.
Navigate to where you want to save the image, then:
sudo dd bs=4M if=/dev/sdb | gzip > rasbian.img.gz
Note: gzip is for compressing the image to a managable size.