Great audio is probably the most difficult, but also most important part of your live stream. Bad audio will drive away your audience and distract from your content.
Anthony Burokas made this presentation at Streaming Media West 2014 covering all the various ways an "Audio Mixing Ninja" gets the best audio possible.
When it comes to live audio for streaming, there are a few additional wrinkles to consider that don't come into play with audio recording when you aren't streaming live.
Streamingmedia.com has a great article featuring audio mixing, and the best practices for your stream. The article covers Fading, Limiting, Compression and more.
"Let’s say my audio is masterful. No hiss. Nice, strong, clean audio. That means I’ve mastered the input trim on the microphone I’m using—especially those on wireless transmitters—and set it so it’s not so loud that the audio clips, and not too quiet that it forces me to increase gain, producing additional hiss."