Simply using register_globals does not necessarily make a script easier to hack, but if it is used in conjunction with some poor coding techniques (basically, using unintialized variables), then it can be a problem.
If you need to use such a script in an environment where register_globals is turned off (which is the default now), the quick fix is to run the [man]import_request_variables/man function before any reference is made to such variables. This will potentially have the same problems as would having register_globals turned on. The real fix therefore is to change all references to get, post, cookie, and session variables to their applicable super-global arrays ($GET, $POST, $COOKIE, and $SESSION) and ensure that register_globals is turned off in your PHP configuration.