i second banjaxedmdt. Incidentaly there is a new Geiger book out which i plan on purchasing. I love Geiger & am currently trying out Geigery art. here's what i've found rendering wise. Build up lots of transparent shadow layres with the airbrush to get a polished look & use colour dodge (if u use photoshop that is) at a very (2-8%)low opacity & build up highlights. Also take a peek at anatomy books - the bone structure diagrams are great for coming up with ideas/combos. also make a colour pallette of dark browns,greys,grey-greens,grey-yellows,grey-blues and other dank colours. On my recent piece, I did the image in black & white first & then used an overlay & soft light layer at low opacity to add colour to areas. Then slowly building these up. Oh & looking at car parts (or my boyfriend's motorbike mags) helps.
Hope that helps :D
http://spacejelly.deviantart.com/
http://rachel-baxter.daportfolio.com/