How can I clean up my MacPorts installation?

The suggestions you received cover what you're after:

sudo port -f clean --all all
sudo port -f uninstall inactive

If you can't wait for the clean one to run in the background, there are a few commands you can run manually and faster.

Remove leftover build files (this is done automatically by default):

sudo rm -rf /opt/local/var/macports/build/*

Remove download files:

sudo rm -rf /opt/local/var/macports/distfiles/*

Remove archives (these aren't created by default):

sudo rm -rf /opt/local/var/macports/packages/*

port uninstall inactive

is the way to go.

I just wanted to add my experiences with the above in case it helps anyone else: When I ran

port clean --all all

it ran for several minutes, and this made me concerned, so I interrupted the process.

I too, wanted to "clean up" after multiple installs of Macports on Yosemite. I had installed first via Terminal, and then did

rm -rf MacPorts-2.3.3 which I realized does not clean up, because I saw the command port stopped working.

My solution: I installed again with the pkg installer, which doesn't leave any directories behind.