Sentences using "which" or "that" to separate statements frequently contain mistakes in the usage of commas. An instance of "which" is preceded by a comma, which is often forgotten. Instances of "that" are often incorrectly preceded by a comma that should not be present. When in doubt, trust your ear. "Which" wants a pause before it and "that" does not.

Use a comma for serial lists before the last item, that is:

Without the final comma, it is hard to tell if the final item is a "bar" or a "bar and baz". (Writers and editors sometimes refer to this punctuation as a "Harvard comma" or "serial comma.")