Better sentence-writing in 30 minutes a day - Dianna Campbell 1995

Compound sentences
Using commas in compound and complex sentences (a review)
Punctuating sentence combinations

As you know, one use of the comma is to punctuate compound and complex sentences. In the last chapter, you worked with compound and complex sentences, but there we were primarily interested in the structure of those sentence forms. Here our emphasis is on the punctuation of compound and complex sentences.

Compound sentences

Here again are the coordinating conjunctions:

The basic rule is this: In a compound sentence, you insert a comma before the conjunction that joins two clauses. Your writing teacher might tell you that sometimes it’s okay to omit the comma if the clauses in a compound sentence are short and if there’s no chance of confusion. That’s true. But for the purpose of simplicity and for reinforcement of your knowledge of the basic rule, you should insert a comma in all compound sentences in this chapter. This is also the simplest rule to remember and use in your own writing.

Also remember that you don’t put a comma before every and or but you see in a sentence. Make sure there’s actually a clause both before and after the conjunction.