If you know the topic or guest ahead of time (a week or two) you can ask listeners to email or post questions they would like to be asked so that you can be sure to ask about what listeners want to know.

Also, you might want to try what Leo Laporte does with some of his podcasts such as TWIT and TWiM and have a pool of rotating guest hosts that appear on a semi-regular basis. This way you can have others on the show, but they dont have to necessarily make a weekly commitment and then when you want to take a break, maybe one of them can fill in for you.

When I started using using WordPress I would not have there would have been enough content to make a weekly podcasts, but the community is so active, there is always something new, whether it is a theme, plugin, WordCamp, or a new development within WordPress/BuddyPress/Automattic. It shows no signs of letting up in the near future.