I wear one with gears and freewheel, but cannot wear one fixed, as it gradually wobbles round onto the crossbar on fast descents. This induces a strange sort of calm terror in my veins and renders me an accident in a holding pattern.
The trick (if there is one) is to wear the strap (very) short and make the shoulders into a special 'courier-bag-restraining' inverted elipsoidally colluded parabola. If you look carefully at your screen, I am making that shape with my spine as I type.
When I was a motorcycle courier (a fact I rarely mention) the coolest of the cool wore them short-strapped and back-to-front so that the company name was invisible. I will not mention which were the coolest machines as this is a cycling thread, but the V50 has to get a mention.