This site has great walkthrough's for the optimistic Industrial design sketcher. You don't neccessarily need those expensive markers to render as they stop and quote every once in a while. Better just stick to the good ol' Ball point and highlighters. I still love going back to my pencil but only for shading surfaces.
Also please check out Scott Robertson, the overlord of surface shading and in general everything related to drawing.
idsketching.com