I have to wear "dress" clothes at work too. I used to roll them up like has been suggested, but they still arrive a little too wrinkled for my taste. About six months ago I found a really nice suit bag that fits on my rack, in fact it's designed to be a commuter bag. My clothes go in it just like a piece of carry-on luggage. It's from two wheel gear. They're on the web. It'll cost you somewhere around a $100 bucks and has been worth every penny. My shoes, clothes, everything fit into it. It really works great, and nobody who doesn't know I ride to work would never know. In fact, I've taken a suit in this thing and it arrives without wrinkles.

You can see it on the back of my bike in the picture.