On a laptop. If you're just recording yourself or conducting an in-person interview, you can use a laptop and a microphone. You'll need an audio application. If you already have one you like, then great--stick with that. Otherwise, I recommend Audacity, a free, open source, cross-platform audio recording and editing tool. Download Audacity and take some time to familiarize yourself with it. Try recording and importing WAV and MP3 files, and cutting and pasting sections around. Also, see if you like how your microphone sounds.
On a portable device. On the street, an iPod or other small recording device is an even more portable way to capture in-person interviews. I've recorded audio on my iPod using Podzilla (see Mod Your Pod, page 135) and using Griffin's iTalk accessory. If you're an old pro, you're probably already sporting a MiniDisc recorder.
I record most things at 44 kHz, 16-bit stereo, but you can choose lower or higher quality to reduce file size or improve the sound. If you think you might want to save your audio to CD or another "audiophile" format later, record at a higher quality; you can always compress or convert it later.
Using Skype. This free internet telephony application is a great way to conduct remote interviews and conference calls for podcasting (and I use it now for most of my regular phone calls as well). Download, install, and sign into Skype. If your interviewee has done the same, the call is free; otherwise you can pay 2 cents per minute to call their regular phone, anywhere in the world.
Using iChat. On the Mac, you can use iChat instead of Skype, but it only supports conference calls in Mac OS 10.4 (Tiger) and above.
Route the output. Now we need to route Skype or iChat's audio output into our recording application, Audacity. Unfortunately, operating systems still have a hard time routing audio between applications, so we'll need to chain them together with a hodgepodge of platform-specific utilities. Given the variety in sound setups, sound cards, input devices, and recording applications, be prepared for a little trial and error.
Some applications need to be launched before others in order to communicate. Unfortunately, this can vary from system to system. So if this Skype--LineIn--Audacity order doesn't work, exit them all and try some launch order variations.
On the PC (Windows XP), we're also going to use Skype and Audacity. See the On a laptop section for Audacity setup (it's the same for PC and Mac). What's different is how to configure Skype and route its audio. Note that the recipe below, which uses XP's Control Panel/Sound Properties window, works for many systems. But with the PC's zillion possible configurations, sound cards, and audio drivers, your mileage may vary.
Try switching the settings before or during a call, and watch the sound input levels in Audacity to detect when the sound is being routed. You can also use VAC (Virtual Audio Cable), which routes audio from all your devices to Audacity or other sound applications. Check out Virtual Audio Cable at spider.nrcde.ru/music/software/eng/vac.html.