I always try to prevent blisters before they occur. I rub my feet all over every morning with Arrid cream antipersperant, let them completely dry, then apply either Avon antiblister balm or body glide beneath all my toes and over the ball of my foot. Before we ever leave home I apply a large piece of moleskin inside the shoes right across where the toes hit. If I have a pair that I know might irritate the top of my foot I'll apply moleskin there too. I prefer to apply moleskin directly to the shoe, rather than my foot. I rub my feet down with peppermint lotion every night and keep a close eye on them. I do carry precut peices of moleskin into the parks with me, but thus far have never had to use them. I also never wear tennis shoes into the parks. I love my Crocs, Reefs, and especially my Teva Mush flip flops. The one trip I did wear tennis shoes I was miserable (feet hot and sore).
FYI: Jafra make a great peppermint kit for your feet. It has peppermint foot lotion, peppermint foot soak, and a cooling peppermint foot/leg mask that's supposed to increase circulation and is absolutely wonderful. I wouldn't dream of going to WDW without my little blow up foot bath and my Jafra peppermint stuff.
Anyway, what I'm saying is take care of your feet before problems ever start.