http://www.vimeo.com/6728285

This tutorial, via TronAudio, looks at how to use compression in Ableton Live:

This is a nice wee video Iv done, to help all of you out there struggling with compression. I explain what a compressor is, how it works, the parameters you need to understand, and various ways and techniques of using one to achieve a variety of results, from Mixing, shaping sounds, sidechaining to FX. Hope its of good use.