* {
  padding: 0;
  border: 0;
  margin: 0;
}

body {  
  background: #729fcf url(bg.png) repeat-x left top;
  text-align: center;
}

#mydiv {
    position:absolute;
    top: 50%;
    left: 50%;
    width:430px;
    height:147px;
    margin-top: -73px; /*set to a negative number 1/2 of your height*/
    margin-left: -215px; /*set to a negative number 1/2 of your width*/
    text-align: center;
}