How do I record decent sound during an interview?
You should definitely get a mixer!

The "complete" setup for audio would include:


a field mixer (like the PSC ProMix just for example) - needs XLR in/out
lavolier microphones
shotgun mic (or other very directional mic) with boom or pistol grip - this way you aren't stuck with the lavoliers all the time!
cables connecting the microphones to the mixer
your recording device
cables connecting the recording device to the mixer
headphones to monitor the sound coming in, and going out, of the mixer


Note I say your recording device, because theoretically you could record to the camera (not recommended).

The monitoring part is pretty crucial, because you don't want to be surprised in postproduction.

I prefer more directional microphones, because lavoliers can pick up some kinds of ambient noise.  Note however: a shotgun requires skill and endurance to use correctly!  Not to mention, an extra person on your crew.

I would also avoid wireless if you can.  It seems cool, but there's always some unexpected source of interference, and it can potentially add a lot of time to your set-up.

I hope this helps!