Large areas, that dont run into adjacent objects (youll see what i mean if you use one Takevin) i use a Tamiya/Olfa cutter/scribe tool, its kinda like a hooked blade, that instead of "scratching" a line into the media, it removes a fine line of it, very neat too.
There are so many tools for this job, Hasegawa Tritool do a killer line...but you may find buying a few various options, for various areas and shapes?
lee