me and some other forum members use a program called ACDsee Canvas
its a mix between a cad and illustrator, that i find extremely user friendly(i picked it up within 2 days of never using it before, no previous cad experience)
when i design a body i take who would be playing into consideration and more or less design a guitar around a persons body, then mess with the design
once im satisfied with a design ill leave it for a few weeks, come back to it, adjust what i dont like anymore and then print it out, transfer it to MDF and then use that as a template for future builds
in the future i would like to model a human body in a 3d cad then work with designs based on that