I switch between using a backpack (I prefer this over a slingtype back, it seems to spread the weight better for me), and a rack back. Rack bags are the best for your body, but then you have to consider the extra inches when maneuvering through traffic.
Does your work have the facilities to store clothing. If so, drive in on Monday with a few days worth of clothes and then drive on Friday to take them home (or ride in on Friday and just shove them in your bag for the ride home--you'll may want to freshen them up over the weekend anyway).
"Bicycling is a big part of the future. It has to be. There's something wrong with a society that drives a car to workout in a gym." -- Bill Nye