It's worth reading this: http://nicolasgallagher.com/about-html-semantics-front-end-architecture/ and this http://coding.smashingmagazine.com/2011/12/12/an-introduction-to-object-oriented-css-oocss/. Also Jonathon Snook's book on SMACSS (in our dropbox library).

While I don't agree with all of their arguments, adopting some of the thinking has really helped me write smarter css - especially Sass, which supports the modular approach really well.