An option that you can only use if you have a main form that loads other forms in its container, that is if your main form is created once and exists forever, is to create the global variables on the main form and access them with get_open_form().global_variable.
A more elegant option is to create a module and call it Globals where you define all your global variables. Then you import that module on all the forms and you have access to all the variables defined in Globals.