Footer bar is not on the bottom

How can i make fix the footer bar on the bottom, regardless monitor resolution and height of page?
The css relative to footer is this. What i must to change?

#footer {
border-top: 1px solid #000;
background: #222;
color: #7B7B7B;
font-size: 13px;
position: relative;
z-index: 100;
clear: both;
padding: 10px 0;
}