Check

• space usage of top pathes

sudo du -sh /* | sort -h

• how much remain

df -h /

• if use btrfs

# as superuser
btrfs filesystem usage /
# as normaluser
btrfs filesystem df /

Clear cache regularly

sudo pacman -Sc

Remove packages that are no longer dependent on them

# list
pacman -Qdt
# recursive uninstall
sudo pacman -Rns $(pacman -Qdtq)