I don’t like the idea of having a display manager like xdm or lightdm as I am the only user of my PC so here is how I autologin.

Edit /etc/gettytab and modify the X entry so it reads

...
X|Xwindow|X Window System:\
    :al=USERNAME:\
    :fd@:nd@:cd@:rw:sp#9600:
...

Edit /etc/ttys to read

...
ttyv8    "/usr/libexec/getty X"    xterm    on    secure
...

Now append this to your ~/.profile (assuming your are using bash as your default shell)

...
if [ `/usr/bin/tty` = '/dev/ttyv8' ]; then
    /usr/local/bin/startx
fi

Finally create a ~/.xinitrc with your chosen desktop manager executable

exec startxfce4