Write everything in terms of how it will sound. In radio, you have four key tools to develop your story: speech, music, sound and silence. Use these in developing plot, characters and setting. Move the plot along with dialogue and narration. Give your characters distinctive voices that the listener can easily identify. Use descriptive narration, dialogue, music and sound effects to help your listeners imagine the setting.
Open your story at a particularly interesting or critical part of the story. Hook the listener, then hold his attention by moving the plot along toward the story's end. Keep your plot focused and linear so that the listener can easily follow it to its resolution.
Write dialogue that sounds like real people talking. People don't talk in complete sentences, and neither should your characters. Give the characters distinctive voices so the audience can easily tell who is speaking. To make sure your dialogue sounds real, say it aloud. Listen to people talk, remember conversations you have heard, and try to write how people speak.
Create a memorable setting with telling details, so that the listener can imagine being there. Create a landscape and an atmosphere for your story. You can make the room sound hollow or have a lonely train whistle blowing in the background. You can use your characters to define the setting with descriptive dialogue.
Keep it personal. When someone is driving down the highway listening to the radio, it's just you and her. Tell your story to one person at a time.
Keep it simple. Unlike print, your listener can't turn back a few pages and reread a passage. Unlike the Internet, your story is not available 24-7. With radio, you have one chance to make an impact on the listener, and he has one chance to get your story. Don't confuse him with five-dollar words and meandering plots. Keep your writing short, tight and focused.