Every year, my students struggle with the concept of when to add a comma before a conjunction. I created this worksheet and it has really helped their understanding of this concept.
In my room, I have a BOAS poster hanging... surrounding the poster are feather boas and snakes. Since I implemented this visual, it has been extremely helpful in assisting the students in remember what conjunctions are.
Whenever students come to a question that asks them whether they need a comma before the conjunction: I have them draw a line before the conjunction word, then read the portion before to see if it is a complete sentence and the portion after to see if it is a complete sentence. If they are both complete sentences, you need a comma. If one or both are fragments then you do not need a comma!