If you don't need it, dump X11. Dump the extra fonts, and the printer drivers you don't need. Dump the desktop pictures and User Pictures in the Library you don't need, then all the widgets and whatever app or utility you don't need like Airport Admin if you never use them. Download DeLocalizer from bombich.com and delete language support unless you speak Urdu or whatever.
Altogether you can save perhaps 3GB... sometimes more.