<!-- The HTML -->
<div id="container">
   <div id="header"></div>
   <div id="body"></div>
   <div id="footer"></div>
</div>

/*** The CSS ***/
html,body {
   margin:0;
   padding:0;
   height:100%;
}
#container {
   min-height:100%;
   position:relative;
}
#header {
   background:#ff0;
   padding:10px;
}
#body {
   padding:10px;
   padding-bottom:60px;   /* Height of the footer */
}
#footer {
   position:absolute;
   bottom:0;
   width:100%;
   height:60px;   /* Height of the footer */
   background:#6cf;
}

Keep the "footer" always at the bottom of the page, with 100% height

100% height, keep the footer at the bottom

CSS