1. Use Components for repetitive objects. The objects in the component are saved only once and all other placements of the component are only references to its location.
2. Reduce Display Resolution of Smooth objects, see Here. You can change multiple objects simultaneously, see Here. Reducing the Display Resolution for a component automatically reduces Display Resolution for all smooth objects inside that component.
3. Keep image map resolution as small as possible for images loaded for Materials, Billboards, Underlays, Backgrounds, etc., see Here.
4. Delete ghosted objects by choosing either Clear All Ghosted or Clear Ghosted Tool Operands from the Edit pull-menu, see Here.
Set File Optimization and Compression settings in the Project Settings dialog. Select Project Settings from the File pull-down menu then click on the Project Files tab.
