The demo you create for your Podcast or Internet Radio show should be no longer than 5 minutes. Most of the time, you won't get more than 30 seconds to make an impression because people who make programming choices either know what they're looking for and judge you against that standard or are listening for something that is so new, fresh, and unique it demands more attention.
If you get past the first 30 seconds and a Program Director listens to all five minutes of your demo, that's great. Trust me: if five minutes isn't enough, he/she will contact you for more.
Since the first 30 or 45 seconds is so important, make sure your demo starts with something that is absolutely riveting and compelling. Find a snippet of audio which showcases your talents or your show in the best light possible. Remember: a demo can be edited together in an audio montage format. It doesn't have to follow the congruency of a standard radio Aircheck.
Label your demo with the Podcast or show name and make sure you include your contact information on it including email, phone number, and website.
Include with your demo a short cover letter and a one-sheeter: all the information that's important about your show on one standard sheet of paper. Besides not having much time to listen to demos, Program Directors don't want to read a long, drawn-out history of what you're doing. Give them the "Who, What, Where, When, and Why". If you have stats on current listenership or any impressive demographic information about your audience include that, too.