Although I'm not left-handed myself (not creative enough, I guess!), I've had to teach a few people how to cast-on and/or knit left-handed. One of my coworkers passed on a great video on how to cast-on left-handed, part of a selection of left-handed knitting videos from the Knitting Site. This is a great improvement on my current method of sitting the person across from me and making them follow along!

• Videos for left-handed knitting - The Knitting Site
• Continental Knitting Instructions - KnitPicks
  Some left-handed people find it easier to knit using the Continental method instead of English 
• Knitting Left-handed - Knitty.com