Hi,
I follow the strategy proposed by
MGMAT in its sentence correction book.
Following are the excerpts on the subject from this book:
Essential vs Non-essential Modifiers (which vs that)
• Which is used to introduce “non-essential modifiers”.
• That is used to introduce “essential”modifiers.
• A modifier introduce by the word which can be removed from the sentence without the sentence losing any essential meaning.
• A modifier introduced by the word that is essential to the meaning of the sentence.
Punctuating essential and non-essential modifiers
• Commas are used to separate non-essential modifiers from the noun that is modified.
Amar