I recently came across a brilliant page on css-tricks.com which details how to make IE-only hacks for CSS. Now under normal circumstances, it’s best to just write your CSS to the lowest common denominator (ideally W3C standards as much as possible), but when you have to lower yourself to using IE7 (and often even IE8), sometimes it’s best to just use the force, and influence the lesser browser to do your bidding.
The link is here, http://css-tricks.com/how-to-create-an-ie-only-stylesheet/. Check it out, and see just how much easier it can make your “compatibility with older browsers”.