A lot of people are plenty content to take their bikes to the local shop every time they need maintenance and tweaking done.  However, maybe it's because I grew up with a gear-head body shop owner for a dad or possibly because I was unduly influenced by Zen and the Art of Motorcycle Maintenance, but I at least like to live under the illusion that I'm DYI.  While I'm still a relative newb, I try to do my own wrench work whenever possible.  More often than not, this means derailleur adjustment.  (Now that my new bikes are quickly turning into an old ones, one of these days I'll get the courage and ambition to learn how to true up a wheel, change the bottom bracket, pack hubs, etc.  But for now, I'm content with the basics.)

Being a visual learner, the first place I turn when I need to learn anything about anything is YouTube.  Here are the two best videos I have found on derailleur adjustment.