Work on your right hand accuracy: make sure you aren't making any excess movement with your slapping hand/arm, and keep everything RELAXED. Also, make sure you aren't beating the living hell out of the string. It only needs to hit the the fret, not grind into it.
And like JB696 said, sub-par slapping technique only makes you look bad if you hang around near some of the gig-less jackasses at Guitar Center.
+1
something I found handy when I first learnt to slap was to set a metronone up VERY SLOWLY and slap each string. E, A, D, G and back down again in time with the beat. Speed up, rinse and repeat. Try it with simple octave slapping and 12 bar blues to get the movement betwen strings (sorry for tab, but can't think of another way to show it)
G|-------------------3--------
D|----5------7-------------3-
A|----------------1--------
E|3------5--------------1---
Then do the same but so that you're popping and slapping on each string
pop each open, hammer on the note then slap the note.
G|--------------------0h2-2-------------------
D|--------------0h2-2-------0h2-2-------------
A|-------0h2-2--------------------0h2-2-------
E|-0h2-2---------------------------------0h2-2
mix it up between the strings to get some right hand movement underway
Find some simple slap tunes (RHCP: can't stop, RATM: take the power back) and noodle away!
Hope that's some use
