If your shoes, or your socks, or BOTH don't fit properly, you're in big trouble. Blisters are caused by friction between the skin on your foot and another surface. The potential for blisters is even greater when there is more pressure caused by a wrinkle in your sock, or a poor fit. Also increases when moisture and heat present.
SO, try these ideas (FYI: I'm not a pediatrist, pedorthist, or any type of foot expert. I just hate blisters. AND I read the DIS boards, too

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Make sure your shoes and socks are the correct size for you and for each other (not just the ones you like the most--remember this is about comfort and health, not fashion!). Try Thorlo socks-get the right size and get them before you get shoes. Then, buy shoes that give you good support, don't rub or pinch AND that will fit over your Thorlo socks. And be sure your shoes fit RIGHT AWAY. Unless you are getting very sturdy and stiff dress shoes, breaking in won't increase actual comfort, or improve the fit.
To prevent moisture, spray your feet thoroughly with spray antiperspirant, let your feet dry and then put on your socks and shoes.
Change shoes and socks (or at least socks) part way through the day.
Remove insoles (if they're removeable) at the end of the day so they and the shoes dry better.
If you do all this and still develope a "hot spot" (beginning of a blister, but not blistered yet), put moleskin over it or around it to protect from further irritation. It also helps to gently massage (don't RUB, but massage) the spots that are sore, but not blistered. That gets the circulation going and helps them heal up quickly.