Here's one way [...among others]
I make a copy of /usr/X11R6/lib/X11/xinit/xinitrc that I name xinitrc.flux,
and I edit it to read
Code:
# Start the window manager
fluxbox
Then I copy /usr/X11R6/bin/startx to /usr/X11R6/bin/startflux and edit it to read
Code:
sysclientrc=/usr/X11R6/lib/X11/xinit/xinitrc.flux
Then from login prompt
startflux