So there are two schools of thought on slapping, one is arm parallel to the strings, the other is arm perpendicular to the strings. I started out in the perpendicular school, and switched probably 9 years into playing, which really fucked me up. My advice when starting is to know which one will work for you, and stick to it. In broad terms, arm perpendicular is the "rock" style, and arm parallel is the "funk" style... part of this has to do with where you sling the bass. For me , I started out playing with my bass body slightly below the belt buckle, and it gradually moved up until now it's mostly over the belt buckle. I also went through a phase where I didn't slap at all for several years... when I started wanting to slap again, I needed to switch styles based on the fact that I wore my instrument differently, and going perpendicular meant contorting myself. The primary difference for me has been how I do my muting while slapping, and the types of lines that I tend to play now. I think that parallel arm works better for the diverse slapping techniques that are out there, but as always YMMV. Conversely, the "Flea" school of rapid fire octaves and such may be a bit easier in perpendicular position.

In either case, the basic idea is to loosen the wrist, and let the thumb almost become a projectile, in the same way that a drummer almost wants to throw the stick at the drum instead of driving it into the drum. Letting your thumb bounce off the string is important in letting the tone develop. Practice doing this on every string with a metronome, strive for rhythmic perfection and striking only one string at a time. Popping is a bit different for parallel vs. perpendicular... with perp. you will hook your finger under the string and it will be perp. to the string as well, so you are using the tip, similar to plucking technique except you are under the string. With parallel, you sort of use the side of the tip ofyour finger, like from the tip to the first joint. Now after you thump with your thumb, pop the octave of that note... keep the thumps in time with the metronome and try to get the pops exactly in between so that you have a steady flow of notes. When you have this basic concept, and it feels good, start mixing it up. A very good book for developing this style is called "Slap It", it's orange colored and pretty thin. It comes with an audio CD (my version came with a tear out piece of vinyl) and has lots and lots of example parts. This is the basis of the Larry Graham/Louis Johnson stuff that we all know and love.

Nowadays, there are some more techniques to go in the pile, pioneers by guys like Marcus Miller and Victor Wooten. Most important of these is the thumb upstroke. What these guys do instead of bouncing the thumb off the string is stroke across the string more like a guitarist playing with a pick, and then comeback with the nail side of the thumb to get two strokes of the same string in one motion. Adding a pop after the upstroke with the thumb gets you three strokes in one, and they might even add a second pop with the midddle finger for four strokes. Practice this all on one note, or across an octave, or any other interval (or combination of intervals) for a beginner course in this technique. Combining this with left hand hammerons (fretting a note with out a right hand attack) and pulloffs (releasing a fretted note while holding a lower (pitchwise) fret on the same string) makes for a hefty rhythmic bag of tricks to pull from. The key with making this stuff sound right imho is A) restraint... Marcus is great at this, he'll throw in a flurry of notes here and there as an accent. and B) staying in the pocket. As much as I am not a fan of the bass heroics of Victor (wrt playing in a band context), there is no denying his extreme level of skill... he may play too many notes (for me) but I dare you to find one that's out of place.

I am not the strongest slapper out there, so I'm leaving a lot unsaid here, and I encourage people to check out some books /videos for more instructions in this area.