#1: Lube-up heavy and often
#2: Don't be cheap when buying shorts & break them in ahead of time
#3: Break in your saddle - I ride a Brookes leather saddle that has a good 1,000 miles on it (any good saddle will do)
#4: Stop every 10 miles on the tow path for a "butt break" - it will preserve the ability to sit in your chair at dinner.
#5: If you do chafe, you can recover with a liberal application of Zink Oxide ASAP.
#6: If you're completely raw, stop and ride the SAG for rest of the day.
Sitting on a bag of ice in Hagerstown is no fun.