change your /etc/inittab to the non-gui runlevel (usually 3, i think). example from my inittab:
# These are the default runlevels in Slackware:
# 0 = halt
# 1 = single user mode
# 2 = unused (but configured the same as runlevel 3)
# 3 = multiuser mode (default Slackware runlevel)
# 4 = X11 with KDM/GDM/XDM (session managers)
# 5 = unused (but configured the same as runlevel 3)
# 6 = reboot
# Default runlevel. (Do not set to 0 or 6)
id:3:initdefault:
notice it says id:3, which as the key says, equals multiuser mode with no graphical login (no KDM/GDM/XDM, etc.)
then modify the .xinitrc file in your /home/user directory (or make the file if you don't have one), and put "exec fluxbox" at the bottom (without the quotes). the next time you reboot or logout, type startx, and that should boot you into fluxbox.
