Here’s a quick link to a Canon flash tutorial written by Charles E. Gardner.
The article teaches you how to properly set up your Canon flash guns for proper lighting.
Almost everything you need to know about using multiple flash units is covered including ratios, how to mix in ambient lighting using slow shutter speeds, proper positioning for flattering portraits (oblique short lighting, full-face short lighting, flash for candid shooting), fill:key combination and other topics.
Thanks to Charles for not leaving any of the juicy bits out.
The article can be easily adapted for use with other flash systems.