When layering different pieces of denim in one outfit, avoid the Texas tuxedo route. Opt for different cuts, washes and textures. Layering denim on denim can actually be fun; not scary.
Also, another quick tip: sizing. If wearing a shorter or very body conscious dress, wear a larger, almost oversized, fitting jacket. Think of it as your boyfriend's jacket. Play with proportions.
Now go have fun! ;)