The usual approach is separating into different CSS files and then use a CSS minification tool for production releases that combines and minifies your CSS, i.e. see minify or YUI Compressor.
You should use different CSS files and move them into 1 file while building your application. There are special tools for this that do just that as this is the only way.
In conjunction with the keyboard shortcut (ctrl+M, ctrl+L) this for me is invaluable. as it instantly reduces your huge, long page that you have to scroll through MUCH, MUCH quicker.
Hope that helps you out !!