Blisters are due to repeated friction, heat, or excessive moisture that can create a superficial sore just beneath the skin.
When the foot rubs against your socks or shoes, the friction created can cause excessive fluid build-up within the upper layers of skin. It is important to keep your feet well hydrated but not overly hydrated. There are ways to prevent the development of blisters, especially if the occurrence is more frequently during warmer months or after excessive walking or running. For instance, one may use a silicone-based cream, petroleum jelly or body glide balm for preventative measures.
It is also important to make sure your socks conform well to your feet, fit snugly and have enough stretch so that they do not crease or bunch together. Synthetic socks should be avoided since they do not allow your feet to breath, as perspiration collects and makes your feet excessively moist. There are a wide range of socks available to the consumer that are designed to keep your feet hydrated, yet not overly moistened, such as Coolmax, Drymax or Under Armour. During days in which you are actively on your feet, one may try wearing two pairs of socks so that friction now shifts from sock-against-foot to more sock-against-sock.
It is also important to break in new shoes by increasing wear time gradually. High heels are notorious for producing blisters on the back of the heels. This is explained by the shift of the weight to the front of the shoe, leaving slippage at the back that rubs at every step. Padding the inside of your shoes to avoid any potential friction points is vital to avoid blistering of the skin. The use of moleskin can also be very helpful in protecting your skin.
If you do not know where your blisters are coming from or if they are appearing in places that seem unusual, it may be time to see an expert to evaluate your feet.