When you put in props and other decorations think of why they got there. For example, boxes and stuff. They are basic props. But think of who put them there and why. How would they stack them? Would they fall over? Etc. It doesn't have to be totally logical, just a little bit logical.
Little scenes that you find tell a story. For example, my map pl_Industry (shameless plug) is a big factory space. There are little places where you find props and overlays that are set up to give the impression that something was going on here. On the roof, there is a nice view of the skybox near a scaffold. So I put a helmet, thermos and a lunch bag, as if that was a spot where a factory worker was having lunch when suddenly Red decided to attack.
That kind of thing.