If you're using a 'C' shell (csh, tcsh, etc):
On the host you're exporting to, type: xhost +
On the host you're exporting from, type: setenv DISPLAY
If you're using a 'sh' shell (sh, bash, ksh, etc):
On the host you're exporting to, type: xhost +
On the host you're exporting from, type: export DISPLAY=