APT repositories for Debian and Ubuntu (maintained by LXDE developers):
Add this line to your /etc/apt/source.list:
deb http://people.linux.org.tw/~andrew/debian/lxde/ ./ (for Debian sid)
deb http://people.linux.org.tw/~pcman/ubuntu/ ./ (for Ubuntu)
apt-get install lxde
Select "LXDE" from GDM / KDM, and login. If you don't have any display manager, run startlxde in your xinitrc.