What also helps a lot is to keep in mind that classes are for classifications (categories) and id's for identifications (unique).

I see a lot of people using classes to create unique styles for specific elements. This will make a mess of your stylesheet.

Also try not to think too much in subcategories (nested classes) because it will remove flexibility from you stylesheet. For example:

  .button {}
  .button.text-center {}

is used to create buttons with centered text. But now you need to create another class for other elements that need centered text. So instead this would be more flexible:

  .button {}
  .text-center {}

In both cases you can use <element class="button text-center">.