I draw it on paper first, then lay out geometric shapes in Inkscape, then if I want it to be extra pretty I mess with it in Gimp. I've seen some awesome 3D GUIs though, so I'm thinking it would be awesome to have a program that could automatically convert 2D vector art into 3D cookie-cutter shapes and tubes. Or maybe just a tutorial on how to do that in Blender, if it's already possible.