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:
Replace Gnome with Fluxbox:
Or comment the Gnome line and add a Fluxbox line:
Save your changes and you're set.
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):
Next time you start your computer, xdm will log you into fluxbox.