I was recently a guest on an hour-long show and I was tired. I had exercised in the morning, did my own radio show, and had to be "up" for my interview. It was a struggle and I felt terrible about it, although the host assured me it was a good show. She's sweet that way.
As Kimberly Henrie wrote in her article for About.com, 11 Dos and Don'ts for Getting Free Radio PromotionRemember that radio is entertainment. If you land an interview be prepared to entertain the audience in addition to informing them. Be over-friendly, over-excited, wildly outrageous, flirtatious, funny, morbid, something that the audience won't expect. And do it in a big way. You have to overdo in radio for the emotion to get across the airwaves. A guest who says they will be entertaining, and doesn't deliver is a big disappointment for a show host/producer.
She continues: Become a frequent caller to your local radio show. Call in with jokes, funny stories, Christmas shopping ideas, road conditions, whatever. As long as you are witty and entertaining, the show's host will probably keep putting you on the air. If you add something to his/her show, he/she will love you for it and be more likely to let you plug your business or occasion.
Did you know that Sean Hannity got his start in radio by constantly calling radio shows and giving his two cents. Turned out the other listeners liked him more than the hosts.