For maximum sound quality I recommend using external microphones that are mounted in stands. Holding a microphone will introduce handling noise that comes through as big thuds. For interview recording in the field (i.e. not in a studio) I recommend using two dynamic microphones such as the Shure SM57 (see Figure 1). These are fairly cheap (under $100) and have good isolation. They are easy to use and hard to break. The similar Shure SM58 (Figure 2) includes a pop filter for voice. Shure itself recommends its SM7B (Figure 3) for podcasting, though it's about four times the price.
Figure 1. The durable Shure SM57 is especially popular for recording electric guitar and snare, but its tough sound makes it a favorite of vocalists as well.
If you plan on recording to a computer, I recommend using a simple setup that includes a USB input box (e.g., Digidesign Mbox 2), two dynamic microphones in short stands, and the shortest XLR cables you can get. If you are going straight to a recording unit like a portable MP3 player, then you will likely need a microphone pre-amp or a portable mixer such as the Behringer UB1202 (Figure 4) to connect the microphone cables and boost the microphone signals to line level.
Another microphone option is a lavaliere ("lav") microphone (Figure 5). These are very small clip-on microphones that attach at the front of a shirt collar. Lavs look better on-screen, which is helpful if you are going to do a video podcast. They're also less intimidating to interviewees, because it's more natural to speak normally than lean into a mic on a stand. The downside is that these microphones are prone to noise as the wearer's clothes move.
Either way you go, you should record at the highest sampling rate your machine can handle. You can always "downsample" when you subsequently generate the MP3 for your podcast. But you can't bring a downsampled source back to life after the fact.
Figure 2 (left). The Shure SM58 has a pop filter and a frequency response tailored to vocals.
Figure 3 (right). For that "FM radio announcer" voice, Shure recommends its SM7B, shown here with its voiceover windscreen (left) and instrument windscreen (right).
Finally, you will want a set of headphones to monitor the signal as you are recording it. If you record something too soft, boosting it later will add too much noise. And if you record something too hot in digital, then you will get a hopelessly ruined signal. In the old days of analog tape, you could overdrive the signal and come up with something cool. If you clip in digital recording, you've lost the signal.
Encourage your guest to wear headphones as well. You don't want to have to worry about riding their gain while trying to listen for interesting follow-up questions. Avoid "open air" headphones, which leak sound that could be picked up by the mics. Headphones with a closed back are the way to go.
Figure 4. The compact Behringer UB102 Mixer includes XLR mic inputs (the three-hole black jacks at top left), preamps, and enough flexibility to route your audio creatively. Behringer and other manufacturers make even smaller mixers as well.
Figure 5. The Sound Professionals SP-CMC-2A lavaliere mics use cardioid capsules, which means they are less sensitive to sounds at the rear of the mic, reducing unwanted room noise. The microphones can be connected directly to portable recorders that offer "plug-in power."