Judging from Conan Drill and The Complete Color Works artbook, Aoyama likes to use:
Tombow Mono plastic erasers for erasing
Pigma brand pens for outlining/drawing (particularly 0.1 mm for characters)
"Coloured ink" (Watercolor? Not sure of the brand) for painting coloured images
Looking at a different picture of a desk he seems to keep rulers, Bamboo brushes, whiteout, and a white paint marker handy.
Method:
Plan -> Pencil -> Outline -> Erase -> Details/screentones/colour (depends on what he's doing)
It seems his assistants tend to help out with the screentoning process.
Colouring style - Since he uses coloured inks to paint with of some sort it looks like he starts with the highlights and ends with the shadows.
Random trivia: The cover image of Conan for the Color Works artbook took Aoyama 2 hours from start to finish.
As for how he actually draws, this video should be useful:
http://www.youtube.com/watch?v=NeHSQaSPql4