I use an Arkel Commuter bag on a rear rack to carry my clothes (business casual pants/shirt/socks/underwear/belt in a grocery plastic bag; I keep shoes at the office.) Also pack a towel/soap & shower sandals. Occasionally I will also transport my laptop, as this Arkel bag has a nice suspended padded sleeve in it for the laptop. I can alos fit a small rain jacket & pack a lunch, but it's a pretty tight fit with all of that
My commute is 16 mi each way, and I try to keep a good pace of about 17-18mph as I consider this my daily exercise, so I wear cycling clothes & shower/change at a nearby gym.
I've been wanting to try the eagle creek pack-it folder though as it seems it would do a good job of keeping the wrinkles away