I keep three suit jackets and two pairs of dress shoes (black and brown) at work and ride in slacks, shirt and tie (and bike shoes). My commute is only 2.8 miles, so I take my regular fixie and manage not to get too sweaty. On really hot days, I carefully fold by dress shirt in one of those little suitcase organizer bags, bring a clean undershirt, and ride in dress slacks and any old t-shirt. It works out OK.