Florian Hollender at the Killer Consultant blog has some good Consulting 101 tips on how to shop for clothes when starting as a new consultant. I've never really commented on dress considerations publicly, but I'll add just one recommendation on dress based on personal experience:
Plan to wear a pair of decent shoes on the plane, and don't just pack dress shoes in your baggage.
I have had to let my boss wear my shoes to a client site when his got lost during international travel. Since he took the nice-looking ones, good thing I wasn't left to wearing a bunch of sneakers to the client site!