I found this article really useful. It contains a lot of details about all the codecs and bitrates of videos that the PSP can play, and then it has some suggestions on programs to use to convert files at the very end. I looked around for a long time for something technical like this explaining why the PSP can play some videos and not others so I wanted to share. If you understand this stuff then you don't have to be limited by the settings in PSP Video 9 and programs like that, which sometimes don't turn out very good.

How to Convert Videos for PSP and PS3