Medium: Photoshop CS3
Time taken: 12+ hours? Or so.
Rough sketch. Neon shows up against the dark background. Yeah, it kind of horrifies me too.
I rough out the shapes with a wide brush to get the colour and most of the shadow, before adding detail.
Most of the stuff in.
The cans; and then a pass to deepen shadows where necessary. This looks like the final picture, but actually isn't--I ran it through some filters to add noise. The effect's kind of subtle, but I like to think that it helps. You can kind of see it in the closeups:
BEFOREAFTER
Also, some colour correction to make it less brown.