Open up ~/Library/Application Support/Skype/shared.xml in a text editor, find the <VoiceEng> section, and add <AGC>0</AGC> and <EC>0</EC>. Like so:
<VoiceENG> <AGC>0</AGC> <EC>0</EC> [One or more <MicVolume>'s. Leave them alone.] </VoiceEng>
The thread is from 2007, but it still works in Skype 5.
This feature is single-handedly responsible for the many noisy podcast interviewers you hear. See, when your channel is quiet, Skype yanks up the gain on your mic, meaning that every little sniffle, sip of coffee, or fan running two rooms away gets picked up while your guest is talking. And then, when you re-enter the conversation, your first few words are way too loud and distorted until Skype re-adjusts the gain.
But not anymore on my podcasts.
To manually adjust your mic gain, open up the “Audio MIDI Setup” application (in /Applications/Utilities) and select the Input tab for your mic.