says something about your subject – after all that’s what this style of photography is all about
adds interest to the shot – as I’ve written in previous tutorials – every element in an image can add or detract from your shots. The environment that you place your subject in needs to provide context and be interest without overwhelming the shot
doesn’t dominate the shot – sometimes the location can dominate the image so much that it distracts your viewer away from your main focal point (the subject). Try to avoid cluttered backgrounds (and foregrounds), colors that are too bright etc. Keep in mind that you might be able to decrease the distractions with clever use of cropping, depth of field and subject placement.