deb http://ftp.debian.org/debian/ experimental main non-free contrib
Next run an update and upgrade:
sudo apt-get update
sudo apt-get dist-upgrade
Finally, grab the XFCE 4.6 packages:
sudo apt-get install xfce4 -t experimental
There are a number of improvements in XFCE 4.6 but the big winner for me is the multiple selection of icons on the Desktop. Something that other DE (including LXDE) have been doing forever.