To some extent that's one of those how long is a piece of string type questions.

The normal thing to follow up from the basic shape is either laying out textures (for simple models) or adding details. 

From the spaceship point of view, there is are a pair of terms - greeble or nurnie, this is the process of adding details to create a sense of scale. many people say nurnie are on real models, greebles in software models, but I've come across the two pretty inter changeably, a web search on tuts for either brings up a lot of results. You will also find many plugins to assist, for example blender has a 'discombobulator' script that adds nurnies to selected model faces. I have a similar greeble script for max. 

To some extent in scifi there are no rules, save often under new names, and experiment!  

For something like a gun quite often a few simple extrusions or fillets adds a huge impact. 

Remember if you are not going to use procedural textures too much detail could give you a real head ache later during UV unwrapping

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.