Hi Sketchy!
I'm a SU user too, so I can understand you problems..
First of all, you have to get yourself very detailed models, to make your render look realistic.
Then, you have to add small but relevant details to the environment (my environment is always architecture so I add architectural details to my scenes, but it depends on what is your aim).
Try to use bump and exponent maps diffusely, because they add realism to all materials.
If I were you, I'd add some more light sources, because light variations make a composition more interesting.
But first of all work on you modeling base: download only GOOD models and add 3D details!