If there is one unique characteristic of radio dramatic writing, the plant must be that attribute. Your students will be tempted to sprinkle their dramas with narration. As a demonstration of planting's importance, I generally begin by disallowing any narrating. Once students get into the planting habit, they feel less compelled to allow an omnipotent voice to interrupt the flow of the plot. Tagging is the frequent use of characters' names in the dialogue.
Characters sprinkle their dialogue with name tags for the other characters.
That way we know who is present and who is talking to whom. This rule is harder for students to remember than the more obvious and plainly necessary planting. When characters exert effort, they show it in their speech. As he says the plant line, he suggests he's trying to get it free at that very moment by showing the effort in his voice. Generally, this means adding some well-placed grunting.