rather use xauth (see "man xauth") and work aver a ssh if you use it over the net. it's far more secure.
to make it permanent anyway (better use "xhost + <machine-name>") locate your X startup script (usually .xsession in your home directory) and add a line:
xhost + <machine-name> &
if you are in some windowmanager like kde or gnome just make a small script and start it with you wm:
in an xterm do:
$ mkdir scripts; cd scripts
$ echo "#!/bin/sh" > xhost_access.sh
$ echo "xhost + <allowed_machine>" >> xhost_access.sh
$ chmod a+x xhost_access.sh
if yuo're in gnome start the gnome-control-center:
doubleclick comlex/advanced > sessions (or so..) > tab startprograms (..or so)
now ad the script from your /home/you/scripts/xhost_access.sh here and finito.
in kde there sure is a quite similar way.