To get a good slap for me, I turn up my mids on my bass a little. One thing I would add to what Uriah has shown in the video is thumbing thru the string like Victor Wooten does and then coming back up and plucking with the thumb. Its like strumming the guitar. It helps with speed and doesn't tire out your forearm, when you play that song that requires alot of slap playing. I'm still learning the technique but its helpful.