At the very basic, and I'm assuming you're using gMax...


Create two dummy parts (ensure that the pivots are aligned correctly), named "interior" and "exterior".  These parts are to be exactly located at your model's 0,0,0 point.

Create your exterior model (all parts that are visible from the outside) as normal, creating the hierarchies, etc.  All your exterior parts will have the "exterior" dummy part as their topmost "parent".

The same for the vc/interior parts, except they will have the "interior" dummy as topmost "parent"..

Thus:

exterior:
|- ----------fuselage
|------------wing
                   |-------gear
                   |---etc

interior
|------------cockpit
                   |----------yoke
                   |---etc

i think felix's method applies for fs 2004.
for fs2002 you group your interior and exterior models and name them, guess what, interior  and exterior 

to keep the model framerate friendly (one always should), delete all parts and surfaces that won't be visible form your "seat" in vc cocpit. they are just dead weight for rendering, and you'll never see them anyway.

-juri