Clean all installed:
gz@osx~# port clean --all installed

Clean all:
gz@osx~# port -f -p clean --all all

Upgrade all installed:
gz@osx~# port -v upgrade installed

Upgrade all outdated packages:
gz@osx~# port -v upgrade outdated

Uninstall all inactive:
gz@osx~# port -vf uninstall inactive

Uninstall all from one category:
gz@osx~# port -v uninstall category:gnome

Upgrade port itself (will do the sync too):
gz@osx~# port -v selfupdate

Check to which package provides the given file:

fmbp16:~ bah$ port provide /opt/local/bin/gdd
/opt/local/bin/gdd is provided by: coreutils