Moleskin is an item that can be found in the foot care portion of most drugstores.
It consists of a taupe-colored padding material backed by a layer of adhesive. You can buy a small roll of the stuff for a few dollars. You cut it to the size you need, peel off the protective backing to expose the adhesive, then press it on to the area of your foot where you may be having problems.
After reading about it in the Unofficial Guide (I'd never heard of the stuff), I bought a roll to bring on our recent trip to WDW. I didn't need it, but DW's little toes were getting red, swollen, and very sore by the end of the second day. Half way through our third day, she decided to try the moleskin out. She cut pieces off and applied them to the sides of her little toes, where it was hurting the most. After putting her shoes back on, she immediately felt the difference.
It didn't magically make all the pain go away, of course, but it kept her toes from being further irritated, and by reapplying it each day, we were able to keep going for another five days. Without the moleskin, she said she would have been down for at least a day to recover.
So, as with this trip, I plan to keep a roll of moleskin and some scissors in my fanny pack just in case.