Or
Set up a hook that clears all but the last cache and the currently installed after every Pacman transaction like so:
sudo micro /etc/pacman.d/hooks/paccache.hook
Paste in the hook:
[Trigger]
Operation = Remove
Operation = Install
Operation = Upgrade
Type = Package
Target = *
[Action]
Description = Keep the last cache and the currently installed.
When = PostTransaction
Exec = /usr/bin/paccache -rk2
You can make the hook as aggressive as you wish. After you have it set up, there is no need to explicitly clear package cache.