This is something that for me was a bit of an A-HA moment. I looked at a bunch of notes, videos etc. and then tried for ages and it wasn't happening, and then one day I did it and it worked and I've never looked back.
Have a look at Anthony Vitti's youTube lessons for basic slapping - he does some good close ups (Vitti tends to slap like Marcus Miller in a downward motion):
http://www.youtube.com/watch?v=_ReAU3u-CP4
Also have a look at MarloweDK's stuff (He slaps more like Louis Johnson or Larry Graham and slaps straight on):
http://www.youtube.com/watch?v=PvuvPfNw2_w
And as some one else mentioned, look at the sticky in this forum on Slapping.
Finally, some things I found (YMMV):
- it's easier to slap the lower strings. Start there.
- you don't have to slap hard at all!!
- you have to get your thumb back out of the way fast.
- bounce the thumb off the string don't smack into it
- best starting position is right at the end of the fret board, but varies. Once you get the hang of it you'll pretty much be able to slap anywhere and will use that to vary your tone.
- higher strings need to be hit harder, and further back towards the bridge
- keep relaxed
- it's a wrist rotation, don't involve your arm very much
These are the things that worked for me, but now it's an unconscious thing. And just about everyone I have seen does it slightly differently.
