Create a "wrapper" div to hold all your content and apply its width property through CSS, then give it a margin:auto attribute
<style type="text/css">
#wrapper {width: 760px;margin:auto}
#content{background-color:red}
</style>
</head>
<body>
<form id="form1" runat="server">
<div id="wrapper">
<div id="content">
content
</div>
</div>
</form>
</body>