Be loud and clear. If you have an accent, it is wise to speak a little slower so as to be clear and easily understood. If you know that you need some help with your grammar, get help. You are often judged by the words that you speak and by the tone of your voice. Imagine that your are speaking to only one person when you are on the air.
Listeners listen that way.
Be a friend of the host and the listeners. If you are an informative and entertaining guest, listeners will often stay riveted to the radio and will not call. Just be your best at all times and say things worth listening to. When you hear the music come up when you are talking, that means the break is coming up or the end of the show is near. That is the time you want to bring your comments to a quick close.