Modhails space hulk project website (click on his name below to check it out) has some totally awe inspiring pieces of 3D terrain for space hulk, and he responded to one of my comments with the below. (edited so it makes a little bit more sense away from the comment discussion)

modhail Says:

I’ll just give a few tips to get you by, until I can start working on the beast again:

-PLAN! I spent 3 months just planning what I wanted/needed, discussing how best to go about it, gathering references, collecting bits to use for scenery.
-Make sure you get your measurements straight! You don’t want to end up with corridor sections that are too large or too small… Don’t forget to figure for the thickness of the details you will add to the walls
-Realise things of this size and detail are long term projects, don’t expect to get it all finished in one summer. I don’t expect to finish this beast before 2010…
-Don’t forget this is fun, don’t let it become a chore. 

As to detailing:
-Get more reference material. The more you have, the more ideas you’ll get. Leaf throught all Games workhop publications you have, looking at artwork and pictures of terrain. Look at industrial architecture, architecture in general, etc. Get the Aliens trilogy and look at all the set photo’s. Do this all with a sketchbook at hand…
-Learn to think in layers and simple shapes. Most detail can be made by putting several layers of material on top of each other, or by combining several simple shapes. Those large vent stack, for example, are just the caps from bottles of sportsdrinks, stuck onto PVC tubing… This is camouflaged by adding some tubes and riveted bands of mayo tube foil…
-Stock up on plastic tubing of different sizes. This projects consumes tubing like crazy!
-Build a stockpile of odd bits and bobs. The most seemingly useless stuff can suddenly become awesome if glued into a space hulk at the right place and painted over. Take for example the vent stacks mentioned earlier. And the moulding along the top of the corridors with the massive tubes? Those are just plastic ice cream sticks…

Some very helpful tips in there. And I will be adding some of the extra bits he talks about to my rather HUGE stockpile of bits and pieces ready for any terrain building I do.

If anyone has any other tips, just drop them in the comment section so we can all share in them.