The answer is \"Socks\"
 

I was reminded of the importance of socks whilst reading the "best shoes for walking thread". If you get fitted for shoes, which everyone should do, you must take into account the socks that you were wearing that day. If you were wearing a thin sock, the shoe is going to feel completely different on your foot than if you were wearing a new thick athletic sock. This is a major cause of people returning shoes or saying that shoes "must be worn to see if they will work for you".

A second tip is for shoes that you've had for a season or so. As shoes wear, they start to widen a bit. If the shoe begins to feel loose, you can find a pair of socks with a bit more material to cover the foot and it will compensate. Don't go for a pair of socks with more material in the toe or heel as it will likely end up squeezing your toes, or causing your heel to sit differently in the shoe.