Density Altitude: The Nine Deadly Sins
You may think that you know all about your aircraft and how it performs under varying conditions. But, I think that you will find this training by Orrin "Kurt" Anderson, an Air Safety Investigator with the National Transportation Safety Board to be a real eye-opener.
If you really want to master your aircraft, or if you are just interested in staying alive, then you need to know this material.
I've also provided links to a Powerpoint presentation on mountain flying and two articles on Density Altitude extracted from the FAA's Aviation News.
Microsoft Powerpoint Viewer (If you don't have Powerpoint, you can use this to view the Powerpoint presentation)