Took a vacation last week at a fitness camp and learned something about preventing blisters that I hadn't heard before and wanted to pass along to fellow travelers. After a week of intense daily hikes of 5-7 miles up/down hills, people following the below two recommendations did not get blisters. 1) Duck tape- If you have places on your foot that are prone to blisters (for me it's my little toes and heals), just put duck tape on those spots. Or if you get a "hot" spot, that might turn into a blister after another day of putting on a lot of miles, use duck tape. 2) There are socks that have two layers (Wright), so the friction occurs between the two layers instead of the sock and your feet.
You can find tiny rolls of duct tape at REI, or just re-wrap duck tape around a pencil so you don't have to take a big roll.
Wright Sox dual layer socks are excellent - also sold at REI. But neater and easier than wrapping up parts of your feet in duct tape is applying Body Glide to your feet, then using the Wright Sox. A stick of Body Glide is not that expensive, easy to pack, and you can find it at REI, camping or sporting goods stores as it's a product used by many runners who need to avoid blisters and chafing.
Duct Tape is a last resort, not a preventive measure. It did get me out of the Grand Canyon once when I made the mistake of wearing the wrong socks on the hike down ( synthetic instead of wool). An intense hike for me is 10 to 15 miles with 3000 to 5000 feet of elevation change. After lots of experience with those, my recommendations are (1) BodyGlide on the feet as suggested by Ceidleh, and (2) Smartwool sox, one good pair with no liners. Wright double-layer sox are OK for shorter walks with shoes that require thinner sox. But even for those, I have found thin Smartwool sox that surpass any synthetic in blister prevention. But everyone is different. Use what works for you.
Three people above recommend Body glide on your feet and so do I. It works. Duck tape is not meant to be used on skin and the adhesive can be irritating, causing more trouble to tender skin. If you want to "tape", better use something made for the purpose, like Moleskin, or surgical tape, or bandaids. And once the blister forms, your best bet is a "Compeed" patch. That is the European name, I don't know what they are called in the US. They are expensive but once applied they stay on several days, even through showers.
I learned a long time ago to use anti-perspirant to keep your feet from getting sweaty. Today sporting goods stores sell a product called "Glide" that is essentially a stick of anti-perspirant/deoderant. Also avoid cotton socks. When cotton gets wet, it loses it's cushion and your feet start to rub on the shoe - the friction gives you the blister. Wool or synthetic won't lose their cushion like cotton (and handwash/hang-dry better than cotton). Duck tape will eliminate the friction too, same principle - just be sure to put it on blister prone areas before you start to get a blister. Once you start getting a blister, you have to eliminate the friction as soon as possible to minimize the damage. Mole skin helps, cut a donut shape out and put it around your blister rather than a patch over your blister. The longer you tough it out, the worse it's going to get (and the longer to recover). Ice is also great to cool the area off, but don't just put your shoe back on and start walking or you'll be right back to where you were.
Backpacker's rule: as soon as you feel a pressure point or any sensitivity in your foot, stop what you are doing and apply moleskin. There are other products available. Moleskin can be cut into small sizes ready for use.
My last two toes on primarily my right foot tend to go under, so I usually wrap a flexible fabric bandage around them and that has been great every time. But the key is to do it before you even start all that walking.