Sometimes I use a headset mic, but the sound quality can be kind of poor (I'm talking the cheap kind you would use for Skype).
Usually I just hook up a camera to the computer via firewire and use a lav or shotgun - record straight into the computer, export the audio and delete the video.
A USB mic would be cool, amybe I'll get one.
Another option if you do a lot of this is an audio recorder, like this or another similar unit
http://www.guitarcenter.com/Edirol-R-09-24-Bit-Wave-MP3-Recorder-241132-i11...