I found this on google
http://www.minddrops.com/Tutorials/ASP/flashmxToASP.htm
FlashKit.com also has a lot of information
keywords you would want to look for would be
LoadVars Class
LoadVars()
sendAndLoad()
There doesn't seem to be alot of tutorials on this that I could find online. I can recommend a really good book that goes into through detail on this, it's the Macromedia Flash MX 2004 Actionscript training from the source.
Mike