I'd highly recommend Cozzano's assassin videos here:
http://www.youtube.com/user/MrCozzano/videos
Some of the information there is outdated, in partiular stuff before February 2010 (when Funcom revamped the sin). Nevertheless, these remain the most instructive pvp videos I have seen. Plus, you get to see some really interesting fights here, which seem to be largely absent from most of the more recent pvp footage.