Use a serial comma in any list when the last items in the list are joined by "and." For example, "I very much like blueberries, cherries, and peaches."
Use a serial comma in any list when the last two items in the list are joined by "or." For example, "You should use ferret, sable, or mongoose brushes to add detail to your oil painting."
Use a serial comma when your list ends with a pair of items joined by "and." For example, "He bought cheese, apples, and bread and butter.
Use a serial comma when your list is composed of separate clauses joined by "and." For example, "Pal ran the sweeper, Suzy washed the dishes, and Vergil wrote a poem."
Use a serial comma if the items in your list are long and complex and joined by conjunctions. For example, "The best way to reach my house is to go west on U.S. Route 57 until you reach the stoplight, or turn left on Warble Road after the little yellow house with the white cupola, or ask my sister to bring you here tonight when she gets off work at seven."
Do not use a serial comma if the items in your list are short and joined by conjunctions. For example, "The best way to reach my house is by U.S. Route 57 or Warble Road or with the help of my sister."
Use a serial comma in front of "etc." and "and so forth." For example, "The cellist played an Adagio, a rondo, a waltz, etc."
Do not use a serial comma when the series uses an ampersand instead of "and." For example, "My legal firm Hodges, Pramptly & Berser got me out of prison fast."