HOW TO HORIZONTALLY CENTER A BLOCK ITEM (LIKE A DIV) OR INLINE ITEM (LIKE TEXT)
Let's say you wanted to center a document on a browser page, so that no matter how big you resize your browser the element is always centered:
    body {
     margin:50px 0px; padding:0px;
     text-align:center;
    }
    #Content {
     width:500px;
     margin:0px auto;
     text-align:left;
     padding:15px;
     border:1px dashed #333;
     background-color:#eee;
    }
write this HTML for centering something horizontally in between your body tags:
<body>
<div id="Content">I am a centered DIV</div>
</body>
This will horizontally center block level elements.  To center text, spans, or other inline elements all you would need to add is:
    #Content {
     width:500px;
     margin:0px auto;
     text-align:center; /* THIS IS THE ONLY CHANGE FROM ABOVE */
     padding:15px;
     border:1px dashed #333;
     background-color:#eee;
    }
HOW TO VERTICALLY CENTER A BLOCK ITEM (LIKE A DIV) OR INLINE ITEM (LIKE TEXT)
Note: Do not use line-height to vertically center elements, as it will only work for one line of text and nothing more. 
As for vertically centering items, there are 3-5 methods you can use depending on what it is you are trying to center.
This site describes each method easily for you:
http://blog.themeforest.net/tutorials/vertical-centering-with-css/
Best of luck!