Farnam Street is featuring a handout given by the late David Foster Wallace to his fiction writing class in 2002. It's titled YOUR LIBERAL-ARTS $ AT WORK and covers five common usages gotchas.
2. And is a conjunction; so is so. Except in dialogue between particular kinds of characters, you never need both conjunctions. "He needed to eat, and so he bought food" is incorrect. In 95% of cases like this, what you want to do is cut the and.