I haven't used it, as there just hasn't been a project with enough lead time to experiment with something lately, but I'm fond of the idea of code generators for CSS, like CleverCSS for Python and Sass for Ruby.

Other than that, the best advice I can offer that I DO follow is that I group all HTML elements together, all IDs together, and then all classes together. Within each of those groupings, everything is in alphabetical order.

  body { foo: bar; }
  form { foo: bar; }
  h1, h2, h3 { foo: bar; }
  input { foo: bar; }
  p { foo: bar; }

  #container {}
  #footer {}
  #nav {}

  .etc {}
  .even {}
  .odd {}

and so forth.