Hi Paul,
in the UME Main screen, when you creating a new user, you will find on the bottom a combobox
"Security Policy". Select the entry "Technical User". Now the user password will never
expire and you will never have to change the password after the specify period.
See also help documentation:
http://help.sap.com/saphelp_nw73ehp1/helpdata/en/49/bf6e8101755d5de10000000a421937/frameset.htm
BR
Pedro