Sometimes I can be a little obsessive in preparing to go somewhere to instruct. I have a simple rule when it comes to how many clothes to bring. Always bring an extra days worth. You never know when you are going to spill your breakfast on your shirt or when you are going to pop a button. That reminds me, a small sewing kit can do wonders.
I like to start off day 1 in a tie. It sets a more professional tone. The rest of the week really depends on the environment. A few weeks ago I stepped in for a friend and instructed her Outlook 2007 Power User class at Indiana State University. I walked in with a tie and the first thing they said to me was to take it off if I like. It's better to be a little over dressed then under dressed. By the way, keep the short sleeves with a tie for the Nerd Herd. Were long sleeves.
A reversible belt also comes in handy. It does not matter if you are packing brown or black/blue pants. Just flip the buckle to the show the appropriate color. Several times I had to make the late night Big Box Mart run because I packed black, but needed brown.
Just remember, if you want to charge that higher rate or get higher evaluation marks from your students, dress for it.