I just did my first commute the other day. 5 miles one way, so not as hardcore as others here. Layering is good on the bike. Dress down a few layers then you think you might need. I was getting a bit sweaty over the first few miles. A rack or basket would help. All I have is a backpack at the moment, so my back was pretty hot. I took breakfast with me and spent a good 20-30 minutes sitting at my desk eating and cooling off before heading down the hall to change. I'm debating baby wipes or just getting a membership at the Y about a 1/4 mile from my office for the summer.
EDIT:
Should have added:
Clothes ironed the night before and rolled in my backpack. Threw some water on my face and wet my hair down, plus threw on some fresh deodorant.