A friend of mine does custom dress shirts (http://propercloth.com/) and one of his solutions (besides the detailed measurements option) was to get base measurements from a whole range of retail shirts and let people pick a shirt that they already owned that fits them well and tweak from there (too long/short, tight in the chest, etc.) and then have a very liberal return policy (at least on the first shirt). It's definitely labor intensive to get started (both building up the measurements database plus learning how to tweak them based on the comments), but it seems to work well.