Low poly works can look good with grainy rendering, but turning down the samples makes the shadows looks horrible. How do I create a grainy effect to my render? It can be in blender or post processing (in photoshop). See work below: 
I want that type of effect and a solution in blender cycles.
These effects are generally achieved in post production, they are mostly render engine independent, so they should work both for Cycles or any other engine without requiring that you re-rendering you image. You can easily tweak the effects in a flexible non destructive way without ruining your work or having to wait for a long time.
These are done through compositing, check the Blender Manual about it. You probably want to look into the Distort type of nodes or Filter nodes.
If non satisfy your particular needs perhaps look into making it in an image editing software like GIMP or Photoshop
