XDM

From Fluxbox-wiki

Open /etc/rc.conf with your favorite text editor, and locate the XSESSION="" line. Note that the file may differ depending the distribution you're using.

Example The line should look something like this:

XSESSION="Gnome"

Replace Gnome with Fluxbox:

XSESSION="fluxbox"

Or comment the Gnome line and add a Fluxbox line:

#XSESSION="Gnome"
XSESSION="fluxbox"

Save your changes and you're set.

Another solution

Edit or create a file called ~/.xsession Add this line to it (you should comment out or remove all lines that seems to be other windowmanagers):

exec startfluxbox

Next time you start your computer, xdm will log you into fluxbox.