do you use Bash? Go to the file .bashrc in your home directory and set the variable, then export it.
DISPLAY=localhost:0.0 ; export DISPLAY
you can use /etc/bashrc if you want to do it for all the users.
You may also want to look in ~/.bash_profile and /etc/profile
EDIT:
function get_xserver ()
{
case $TERM in
xterm )
XSERVER=$(who am i | awk '{print $NF}' | tr -d ')''(' )
XSERVER=${XSERVER%%:*}
;;
aterm | rxvt)
;;
esac
}
if [ -z ${DISPLAY:=""} ]; then
get_xserver
if [[ -z ${XSERVER} || ${XSERVER} == $(hostname) || \
${XSERVER} == "unix" ]]; then
DISPLAY=":0.0" # Display on local host.
else
DISPLAY=${XSERVER}:0.0 # Display on remote host.
fi
fi
export DISPLAY