I've been working on terrain a lot lately. There are a lot of great new kits from Games Workshop out, and I'm lucky enough to get copies from the local store in exchange for painting their copies. Not a bad gig, eh?
When I paint terrain, I always take the approach that the terrain should supplement the appearance of the models interacting with it, rather than stand out as a work of art in and of itself. So I tend to go a little light on detail work, and focus on broad gradients and letting the sculpted detail on the piece draw the eye to the more carefully painted miniatures on it. I've found that an airbrush is indispensable in doing terrain work-- without it, even simple pieces take forever, and the natural gradient an airbrush creates makes it possible to do very simple weathering and effects on terrain without spending more than a few minutes on each model.
First off, I've painted the Aquila Strongpoint. The turret comes off in case anyone ever wants to use the thing as a vortex missile strongpoint instead. But I figure 99% of the time, the thing'll just be used as a ruin or an intact building with a dilapidated gun, and in that case, the macro cannon is much cooler looking.
I've also begun work on a pair of complete Wall of Martyrs sets-- I've only finished one trench section so far. All the detail is painted, but the eye still naturally goes to the Space Marines manning the trench, I think-- which is important in terrain work, in my opinion-- don't want people paying more attention to the trench line than the models fighting over it!
I also painted up a couple new Librarians for the Lions Rampant, so that I could field a Librarius in Apocalypse if I ever got the urge to. Librarians of my Chapter do not wear Chapter livery-- they wear the Codex-required blue armor, but do not bear the Lion Rampant or the fleur de lys, and if anything appears on their shoulder pads, it's phrases like "witch."