Center a DIV with CSS

#idOfTheDiv {
   width: 400px; /* here you put the width that you need */
   height: 200px; /* here you put the height that you need */
   position:absolute;
   left:50%;
   top:50%;
   margin-left:-200px; /* this number always to be the width divided two in negative */
   margin-top:-100px; /* this number always to be the height divided two in negative */
}