One of the easiest ways to add realism to your scenery is to add bits of litter and trash. Here's a quick tutorial for adding newspaper litter to your terrain:
Making the Newspapers
To make the papers, I recommend looking around online for generic layout templates, and then modify them in Photoshop to add pictures and unique headlines. They are small enough that the print "text" is just tiny rows of lines, and the pictures are little, illegible blobs and shapes. They are going to be ripped and crumpled, so don't get carried away with making things too precise or detailed. When making these for the Marvel Crisis Protocol subway station, I added a "Daily Bugle" title, and some "super-hero-y" headlines like "Strange Lightning Storm Over City."
Make a few different newspaper pages and then repeat them across the page. When printing them, make sure they are sized appropriately for the scale of your miniatures game. This is simply a matter of eyeballing it with a model and a paper to see if it looks right. If the first page is off, increase or decrease the print scaling as necessary. Again, because the papers will be crumpled and ripped, the scale doesn't need to be perfect.
Once you have printed out a sheet of newspapers, use a hobby knife and a steel ruler to cut them out.
Here are the papers, ready to go, including a few that I ripped in half. Don't worry about the fact that they are only one-sided; they will be glued face-up or folded in a way that doesn't show much of the back side.
Attaching the Newspaper Litter
Adding the litter should be the final step, after the terrain piece has been completely painted, because the newspapers will not be painted, and will be difficult to paint around.
Use waterproof wood glue, and thin it with a little water. The consistency should be just a little thinner than melted ice cream. Set up your work station with a paper towel, tweezers, the cut newspapers, and the glue mix.
Pick one of the papers and crush it into a crumpled shape, imagining how it will lay on the terrain.
Then, use the tweezers to dip it in the glue mix.
Soak it for a few seconds and then blot some of the glue on the paper towel. Don't absorb all of the glue off of the paper, just get rid of any big drips.
Next, press the paper into place on your terrain. Since the paper is wet, you can manipulate it a little to get it in just the right shape. Using a toothpick with your other hand will give you a little more control as you press and shape the paper. Take care to keep the unprinted side of the paper concealed as much as possible.
Add as many papers as you like, ripping and overlapping pages as you see fit. Remember that papers will usually collect the most in out-of-the-way places, like corners and crevices.
As the glue dries, it will bond the paper to the surface and give it some rigidity. Using wood glue instead of white glue also provides the old newspapers with their slightly yellowed hue.
Here is a before and after shot of the subway tracks with the newspaper litter:
Apply as few, or as many newspapers as you like. There's really no "wrong" way to do it. Adding newspaper litter is also a great way to revisit old scenery pieces, and reinvigorate them with a little extra detail.