Plan to wear and carry as little clothing as possible, bearing in mind that hike day may be hot or chilly or rainy or windy—or any combination thereof—and will almost certainly turn chilly at night.
Consider taking rain gear (a poncho may suffice), a sweater (lightweight wool or polypropylene), a brimmed hat, and a change of shirt and socks.
Use the training hikes to figure out what's best for you. You may also want to send along a fresh pair of clothing to change into with those who meet you at the Bolivar Community Center in Harper's Ferry, WV (the end point).
Avoid cotton if possible. Good clothing to wear includes materials that will wick sweat off your body and keep you as dry as possible.