Anyways, i´m still trying to get the tunings right. That is one hard task to acomplish.
There are some manuals around the web. Have you got feedback sent to the computer so you can see the response and measure overshoot, settling time, etc, etc??
I find this one to be acceptable. http://www.expertune.com/tutor.html
Wikipedia's article is a good reference for this also.
Also, if you go through ATMEL's application notes, they have application note 221 for AVR that is the implementation of a PID in C for AVR. You can get some info from that too.