I have always put something like this in my .xinitrc file...
exec dbus-launch startfluxbox &>>/dev/null
The redirect is just to keep less-than-useful messages off my login console screen which becomes visible for a moment when I shutdown or reboot (I don't like seeing those, even for a second).
P.S.: I don't use a display manager with my BLFS system. I used XDM in Arch. For that, I used the .xsession file copied from /etc/skel and .xinitrc as shown above (except with startxfce4 for XFCE). But if I remember correctly, the .xsession file merely executed the .xinitrc file. I don't think my .xsession file had anything else in it.