Say you have a rule that works fine in FireFox and Opera but it's a little off in Internet Explorer..
Keep your styles optimized for Firefox, but in the markup add this:
<code lang="Generic">
<!--[if IE]>
<style type="text/css">
.yourclass {
width:400px;
}
<![endif]-->
</code>
Where .yourclass is the offending item.

You can do more, 
<code lang="Generic"><!--[if IE gte 5.5]> </code>
(If Internet Explorer greater than or equal to version 5.5.)
you can also use lt, lte, gte and ! (for 'not')