Edit /etc/apt/sources.list add these lines:
deb http://www.os-cillation.de/debian binary/
deb-src http://www.os-cillation.de/debian source/

open konsole:
su
apt-get update
apt-get install -t testing xfce4
apt-get install -t testing xfld-desktop

Done.
If you didn't get any Session for Xfce4 in KDM or GDM, do as follows:
Open /usr/share/apps/kdm/sessions folder and copy kde.desktop and rename to xfce4.desktop.
Edit that xfce4.desktop:
Code:
[Desktop Entry]
Encoding=UTF-8
# The names/descriptions should really be better
Name=Xfce4 Session
Name[fr]=Session Xfce 4
Name[he]=Xfce 4
Name[ko]=Xfce 4 세션
Name[nl]=Xfce 4 Sessie
Name[zh_TW]=Xfce 4 工作階段
Comment=Use this session to run Xfce4 as your desktop environment
Comment[fr]=Sélectionner cette session pour utiliser Xfce comme environnment graphique
Comment[he]=השתמש בזה כד×(tm) להפע×(tm)ל ×�ת Xfce כשולחן העובדה שלך
Comment[ko]=사용ìž� ë�°ìŠ¤í�¬íƒ‘ í(tm)˜ê²½ìœ¼ë¡œ Xfce를 ì�´ìš©í•©ë‹ˆë‹¤.
Comment[nl]=Deze optie start de Xfce 4 Desktop Environment
Comment[zh_TW]=使用é€(tm)個工作階段載入 Xfce 4 作為您的桌é�¢ç’°å¢ƒ
Exec=startxfce4
Icon=
Type=Application
When done,......copy this file to : /usr/share/apps/kdm/sessions;/etc/dm/Sessions/ and /usr/share/xsessions.
Now you should have Xfce4 in your Kdm or Gdm.