As requested, I'm listing my favorite science podcasts. I'm definitely not the expert on science podcasts, since I only listen to a few, so I'm also adding a couple of directories that might be helpful.
NPR: Environment is a weekly compilation of their environment stories. They also have NPR: Health and Science, and a ton of other sciencey podcasts. NPR has pretty quality stuff, and lists a lot of local affiliates programs. NPR also puts out Hmmm... Krulwich on Science, which is a weekly random sciency tidbit.
My other source is Seed magazine. They make a weekly science news podcast, "I can't believe it's science", which is funny and entertaining, and sciency to an extent.
There are tons more, and many of them are conveniently listed at The Scientist's guide to science podcasts. (Let me know if that link requires a login, and I'll do something plagaristic about it.)
This may be an obvious thing to say, but this is how I choose podcasts. I search the iTunes podcast listing for things I might find interesting. I've had the most success with the public radio section and just browsing the recommended and advertised podcasts. I'll usually find several that sound interesting, and I download the latest one for listening. If I find myself wanting to fast forward through a particular podcast, I make a mental note not to subscribe to it. When I don't have enough podcasts to fill my listening time, I go searching for more.