apt-get install xubuntu-desktop
- Log out, log back in, selecting Xubuntu/Xfce as my interface
- Chill out for a few days. See that things still work. There will be too many applications (eg there will be both GNOME's and Xfce's default applications available but you want to make sure you'll have everything you need when you remove GNOME/Unity/Whatever.
apt-get remove ubuntu-desktop
apt-get autoremove