Exactly Center an Image/Div Horizontally and Vertically

.center {
   width: 300px;
   height: 300px;
   position: absolute;
   left: 50%;
   top: 50%;
   margin-left: -150px;
   margin-top: -150px;
}

Negative margins are exactly half the height and width, which pull the element back into perfect center. Only works with elements of a fixed height/width.