Well, if you have a few classes then use it like this:
<!--[if IE]>
DIV.Style_1 { blah blah blah }
DIV.Style_2 { blah blah blah }
<![endif]-->
But if you have lots of them, use stylelesheets instead:
<!--[if IE]>
<link href="css/styles_1.css" rel="stylesheet" type="text/css" />
<link href="css/styles_2.css" rel="stylesheet" type="text/css" />
<![endif]-->
... and so on
