I only have 20 frets, so I end up slapping between the pickup and the end of the fret board - I find it gives me more bounce off the string.
John-Paul - do you mean you can't get the sound, or it's just hurting? When I slap it doesn't hurt at all... I slap with the knuckle of my thumb - you need to slap the string and bounce it back off straight away - you get the *twang* sound then. You've gotta use your arm, wrist and thumb to get the right movement...
I slap like this:
* Rest my arm (just under my elbow) on the body of the bass, use that as a hinge
* Bring my arm out, turning my wrist to face the head / nut. Keep my fingers curled in (NOT a fist shape, just bend your fingers to the knuckle). This keeps them available for any pops you want in there.
* Bring my arm into the string I'm slapping, twisting my wrist / thumb back to be parallel with the string - I've heard of people that keep it parallel, but I keep my thumb pointing at a 45 degree angle downwards (My thumb is perfectly straight, there's no bend in it like most peoples!)
* Literally slap the string with the knuckle of your thumb - bring it down onto the string, hit it and rebound off. It's never hurt me - you must be doing something /really/ wrong for it to hurt?!
* After the rebound, you're ready for another attack. Be prepared for it to take ages to perfect... try slapping each string for 30 mins at a time - that's how I got semi-decent at it! Practice, practice, practice!
Obviously what I do won't be the same as everyone else, and it might not even be the most efficient way to do it! But I thought I'd give you the low-down

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