#1 If you get a pedicure before you go, make absolutely sure that they do not touch your calluses. You will need those calluses down there.
#2. Wear the most comfortable pair of shoes you own. Alot of people on here swear by good tennis shoes. Not me! I can't stand those hot, heavy things down there. Others love
Crocs and while they are comfortable, I always wind up in either my Teva Mush Flip flops or my Montrail Molokini flip flops. They are the most comfortable pair of shoes I own and I own several pairs of them in different colors. If you think the thingy between the toes on your flip flops might bother you, cover it with moleskin or make sure your flip flops are like the Teva Mush or Montrail Molokini and have a
wide ribbon between the toes (not a narrow ribbon or a plastic thingy). Reef flip flops are also fairly comfortable down there, but not as good as the Teva Mush or Montrail's
#3 BodyGlide antichafing gel is the best thing since velcro. I used to always get chafing between my legs and an occasional foot blister, but have not had a problem with either since using BodyGlide. You can order it online or get it at some sporting goods stores. Bandade also makes an antiblister balm, as does I believe Dr Scholls. Monistat makes an antichafing gel that's good or you can use plain old antipsersperant (not deodorant) on both your feet and inner thighs to prevent blisters and chafing. Oh, yes Avon also makes an antiblister balm that is very good. I would never consider going to WDW without at least a couple of these.
#4. If you do start to develop a blister apply moleskin ASAP (carry precut pieces with holes cut out of the middle of them to the park with you in a snack size ziplock). Or if you know you usually get a blister in a particular place you can apply moleskin beforehand. What I have done is stick the moleskin to the footbed of the shoe in the place where I think I even might get a blister before I ever leave home, so I have a cushion. The best preventative for me though is to just make sure I wear my flip flops. Since using BodyGlide I've never had to use moleskin again, but always carry some just in case.
#5 Carry at least 2-3 pairs of shoes and change out everyday.
#6 Take Motrin every night and every morning and massage your feet with peppermint lotion every night checking for any hot/sore spots.
#7 The only place my feet ever swell is in Florida, so for my next trip I have bought some Elemis Instant Refreshing gel. It's supposed to be wonderful for sore muscles, swollen feet, and even headaches and tight neck and shoulder mucles. It gets good reviews by the folks on cruisecritic.com. I got mine on ebay much cheaper than it lists for. I also have a foot/leg cooling mask from Jafra that I've used down there a few times. It makes your feet/legs feel wonderful after a hard day at the parks.
#8 Try to avoid sodas and other sources of sodium down there. If your feet swell, the skin over the swelling will be thinner and more prone to breaking down (i.e. blisters).
Take very good care of your feet down there! FYI: I started being so particular about chafing and blisters after a trip to San Antonio several years ago. After three days at Fiesta Texas Six Flags, Sea World, and the Children's Museum, I was in agony trying to keep up with my seven year old GD who I had taken down there for a GM-GD trip. When I got back I could barely walk and the chafing between my legs was bleeding, it was so bad. Note to self: never get out of the wave pool and wear a wet bathing suit and wet shorts around Fiesta Texas for several hours in July. The seam of the wet shorts WILL chafe you raw.
)
). They are just darn comfortable. I have never gotten blisters wearing them with a pair of ankle sport socks. On our first trip I wore my Teva's for a day and couldn't stand the kids constantly stepping on my feet while we were standing in line. Not to mention my DH's big hooves are attracted to my tootsies like white on rice when I wear them. 
