[Afro Joe]

muting is one of the most important things about slap bass, if not most important. getting every technique at a consistent volume is too. as in, left hand slaps need to be just as loud as ghost notes using the thumb or it will sound whack.
the drummer thing is a good way too, especially if you have a drum machine or easy access to a set of drum samples. helps to get different grooves sorted out.
slap bass is also one of those things that can be over-used like hell too, and just sound baws. it gets on my tits when i see funk bands and the bass player is just slapping away cos that's what you're 'supposed to do in a funk band'. dinghys.

listen to larry graham, stanley clarke, marcus miller, louis johnson, les claypool and bootsy collins. sorry if you do already, and i sound like a dick.