Many people omit a comma before the and/but/or, but using it consistently can solve some fairly common problems. For instance:
His usual breakfast consisted of cereal, milk, toast and jam, and coffee. [or cereal, coffee, and toast and jam]
Without the comma, how do you deal with a double item like "toast and jam"??
Having so many children you don't know what to do,
sleeping soundly under the haystack, jumping over
moon, losing your sheep, and putting pussy in the
all unadvisable activities.
Notice that there is no comma after "well"! (never S,V)
He hates to sleep in his clothes, to leave his hair uncombed, or to go without breakfast; he prefers showering regularly, being well groomed, and eating on time.
She believes that life is easy, that work is for other people, and that she is a very special person.