You can use something like this:
<!--[if lt IE 7]> <html class="ie6"> <![endif]-->
<!--[if IE 7]> <html class="ie7"> <![endif]-->
<!--[if IE 8]> <html class="ie8"> <![endif]-->
<!--[if gt IE 8]><!--> <html> <!--<![endif]-->
And then target your selectors by:
.ie8 .your-selector
As described here.
If you want to have a switch you can also at a class to the html-tag, e.g.
<!--[if gt IE 8]><!--> <html class ="i-am-so-happy-it-is-no-ancient-ie"><!--<![endif]-->
and use it like this:
.i-am-so-happy-it-is-no-ancient-ie .your-selector{...}
.ie6 .your-selector {...}
Edit
I forgot IE 9.. Acutally Paul Irish suggests to use this form (slightly adjusted by me to make a non ie-switch possible using a single .ie .class - although you have to be aware that ie 6 doesn't support multiple classes, but you get the idea):
<!--[if lt IE 7 ]> <html class="ie6 ie"> <![endif]-->
<!--[if IE 7 ]> <html class="ie7 ie"> <![endif]-->
<!--[if IE 8 ]> <html class="ie8 ie"> <![endif]-->
<!--[if IE 9 ]> <html class="ie9 ie"> <![endif]-->
<!--[if (gt IE 9)|!(IE)]><!--> <html class="no-ie"> <!--<![endif]-->
For an explanation why, see the link above.