Spring cleaning isn't just a thing for your home. Put the duster, broom and scrubber away and trade it in for a keyboard, mouse and monitor. It's time to clean-up your website. Polishing your website requires lots of attention to detail. The process is not a hard task, it just takes a fair bit of time and due diligence. If you take care of your website, chances are Google and Bing will notice! My recommendation is to clean-up CSS often rather than letting it build-up.
Dust-Me is a Firefox extension that finds unused CSS selectors for a given document or website. CSS is one of those elements that can build-up a lot of unused code. That unwanted code degrades performance and complicates updating your site. Say you have a client's website that has quite a few different CSS files and you're not sure what you can remove — sound familiar? With Dust-Me you can turn on the extension, crawl the entire site, then see which CSS selectors weren't used, and then remove them.
This could also be useful if you wanted to split up the styles sheets of a given website into logical sections. First extract any common css into a separate file (you should already be doing this), then run this tool on your section to see what your file would look like.
This would help you get from a single, difficult to manage site.css file to a more maintainable file structure like homepage.css, header.css, footer.css, section.css, etc. Coupled with CSS minification / concatenation (you should be doing this as well!) you can take an existing site's bloated CSS file and split it up into logical sections while still maintaining fewer http web requests. A general rule of thumb is that you should only have 1-3 external stylesheets — typically one will do the trick if there aren't wildly differing styles between sections.