One of the useful tricks I learned from Vasili Zorin a couple of years ago is to supplement my work with all sorts of detailed mini-drawings that make it look richer. Well, in my case it's stickers, graffiti and posters on the walls of my locations.
The trick is that it is much easier to draw such pictures separately and then use the distortion tool in Photoshop to "glue" them on. Something like texturing technique or photobash.
I attach my two locations and posters, drawn separately. Look closely and you'll see where they are located.