Yesterday, I got my root partition almost full. Taking a quick look to the disk I discovered that most space was dedicated to cached packages and to journaling files.
So, in few steps, and withut deleting important stuff, I was able to recover something like 15 GB in a partition of 40 GB.
Not bad, uh? ;)

I’m going to write here the steps I found very usefull.

Clean repos

This command will clean all packages cache pacman -Scc

Resize (if not done yet) the journaling

Edit the conf file

vi /etc/systemd/journald.conf

and set the maximum disk usage to, say, 50M (the default is 10% of the “/” partition, 4GB in my case!! :O)

SystemMaxUse=50M

Restart the service

systemctl restart systemd-journald.service

Sources:
https://wiki.archlinux.org/index.php/Pacman#Cleaning_the_package_cache
https://wiki.archlinux.org/index.php/Systemd#Journal_size_limit