Blister Prevention
Most runners should not get blisters. There are those who are running in situations that make blisters very hard to avoid (Badwater - http://en.wikipedia.org/wiki/Badwater_Ultramarathon), but for distances of 50 miles or less, blisters should not be an issue. I suffer from a skin condition ("non-Hallopeau-Siemens recessive dystrophic epidermolysis bullosa") that makes my skin blister easily, so if I can run 100+ miles without blisters, you should be able to! There are a number of approaches to preventing blisters.
Contents
1 The unholy trinity of blisters
Three factors work together to cause blisters; heat, moisture and friction. Friction is the biggest cause, but heat and moisture make the situation worse.
2 Shoe Fit
This is the most critical aspect of preventing blisters. If you are getting blisters, this should be the first thing to check. Your shoe should fit without any excess slipping. You can't tighten up a shoe to compensate for being too big. If you get black toenails, or blisters on the ends of your toes, it's probably because your toes are hitting the end of the shoe. Either the shoe is too small, or so lose your feet are slipping forward.
Remember that losing or gaining body fat can change the size of your feet. Shoes that used to fit you perfectly may not longer fit the same.
Calluses are a sign of poor fit. A callus is a spot that is likely to turn into a blister with longer distances. You can file down a callus and keep is moisturized, but that is a symptomatic approach. The key is to fix the underlying problem, which is normally the fit of your shoes.
3 Shoe style and quality
A shoe can fit correctly, but cause blisters due to manufacturing problems. A bad seam can rub and cause blisters. A shoe that bends in the wrong place can have a crease that causes a blister as well. Also, check that your insoles do not slip. Some double sided sticky tape between the shoe and the insole can prevent this.
4 Socks
Read about Blister Prevention and Socks
5 Hyponatremia
Hyponatremia is a very dangerous condition. One of the symptoms is swelling of the hands and feet, which can cause black toenails or blisters. Read up on Practical Hydration and Hydration 101
6 Lubricants and Powders
To reduce friction, you can use a lubricant on your feet. A powder can act as both a lubricant and a drying agent. I like Hydropel, which lubricates and helps prevent the skin absorbing moisture; it's particularly useful in wet conditions. (See <here>.) Blister Shield powder is Teflon, which I found helps a bit, but tends to form clumps when wet. http://www.zombierunner.com/store/categories/foot_care/powders/ and http://www.zombierunner.com/store/categories/foot_care/lubricants/
7 Taping
If you have serious problems, it can help to cover the area with tape. Many people recommend duct tape, but it does not breathe, and the adhesive is not very strong when wet for a protracted period. I have found Micropore with tincture of benzoin, which is an adhesive promoter. Check out http://www.zombierunner.com/store/categories/foot_care/tapes/ and http://www.zombierunner.com/store/categories/foot_care/adhesives/
8 Skin Toughening
See Toughening Feet
9 Read the foot care bible
John Vonhof's book 'Fixing your Feet' is invaluable for dealing with blisters. Check out http://www.fixingyourfeet.com/