Here's a great video from YouTube of Ray Brown talking about the history of bass playing. Lots of players in the roots/rockabilly genres forget that jazz layers used slap techniques, refining them beyond their original incarnations. In this video (one of nine videos of Ray teaching a masters class) Ray talks about the history of playing technique and does some great slapping. Particularly interesting is the speed and smoothness of his slaps. I love the flow of his arm motion parallel to the strings.