RULE: Before that -> NO COMMA
Examples: The face that she made upon the news spoke volumes.
She said that she could make it to the meeting without problems.
You have to put salt in the marinara sauce so the flavour comes out. (That's right, there is no 'that'. The 'that' is implied: You have to put salt in the marinara sauce so that the flavour comes out.)
There. Another potentially fatal comma eliminated.