This is my first creation for BlendSwap.
The Dirty Lens Texture is packed into the .blend file and I also put it into the .zip archive. + another Hexagonal Bokeh Texture.
I created the Textures with Gimp.

It's easy to just overlay a transparent Dirt-Image but in "real life" the dirt is only visible at bright areas like reflections and lamps.

It's a very simple Node Setup and beginners should be able to follow the steps.
If you are woundering about all the "Reroute" Nodes, it's because I like simple and clear layouts. :)
I hope you like it!
Sorry for my bad English.