A tip for bassists: It's pretty simple to slap at a sustained speed of at least sixteenth notes. Relax your entire arm, or at least from the elbow down. Then rapidly rotate your hand from left to right. That's the basic motion that you probably have down already; the key is absolute relaxation of that arm. The speed of that basic motion is the easy part; what you need to develop the most at that part is accuracy; you have to master striking one string without accidentally hitting any others. This is hardest on a bass with five or more strings. Bill "The Buddha" Dickens slaps like crazy on a seven and nine string! He's a maniac; do not try this at home.