If, for some reason, you feel you know what you're getting into and have intentionally used markup syntax that's triggering quirks mode in IE (or you just can't figure out why, or maybe your client insists on designing for IE5.x for Windows), then it's time for some hacks.
The cleanest hack is the !important hack. I like it because it lets CSS still render as valid. However, you should note that the !important value is the valid syntax and meant to be used as an accessibility feature of CSS. It's not a value that was ever meant to affect the design.
The fact that IE does not recognize it is a bug and, though it's very simple and easy to implement, it's not recommended to be used liberally as a design fix. The understanding is, eventually (or already), IE will fix this bug so that it adheres to accessibility standards and then your hack will no longer work (especially if IE doesn't change anything about how it runs in quirks mode). In fact, this bug is supposed to be...