I commute on several different bikes. Generally I carry my gear in a Carradice Barley seatbag, which has enough room for a clean shirt, t-shirt, underwear, lunch, wallet, tools, tire-repair gear, etc. I leave a bunch of clothes and supplies at work, such as shoes, slacks, towel, shampoo and extra shirts -- which I replenish on days that I drive. A couple of my bikes have rear racks, and I carry my gear in a Banjo Bros rack top bag when riding those bikes. You can't totally prevent wrinkling when carrying clothes to work, but I've found that rolling my shirts up makes them fit better and they don't seem to wrinkle as bad. Some commuters use panniers to carry their gear, but I don't need that much space since I leave so much clothing at work. I'm also fortunate in that we have a locker room with a shower, and a storage room where I can keep my spare clothes.