How do I get ‘footer’ content on a master page to push down when main content doesn't fill a page

<%@ Master Language="C#" AutoEventWireup="true" CodeFile="MasterPage.master.cs" Inherits="MasterPage" %>
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server" style="position:relative">
    <title>Masterpage</title>
    <asp:ContentPlaceHolder id="head" runat="server">
    </asp:ContentPlaceHolder>
</head>
<body>
    <form id="main" runat="server" style="position:relative">
    <div>
        <asp:ContentPlaceHolder id="MainContent" runat="server">
        </asp:ContentPlaceHolder>
    </div>
    </form>
    <br style="clear:both" />
    <form id="footer" style="position:relative">
    <div>
        <center style="font-size:small">Footer content</center>
    </div>
    </form>
</body>
</html>

bottom:0px;
position:absolute;