I think it's unfortunate that people use the term "slap," because that's not really what you're doing and IME beginning slappers think you're supposed to play hard. Larry Graham, inventor of the technique, calls it "thumping," which I think is more accurate. You really just need to use as much pressure as, say, opening a can of soda by pulling on the tab when you "pop," at the
most, and as much pressure with your thumb when you slap as you would, say, patting the seat next to you to indicate an invitation for someone to sit. It should be a fluid motion that you can repeat all night with ease.
It's especially important to practice keeping the pressure consistent, because when you're nervous, you'll tend to dig in harder (e.g. in front of an audience). This not only causes pain but bad intonation as well as pitch & timing accuracy problems. Your hands should be very relaxed when you play. Think of the way a cat walks - it seems effortless. That's how playing your instrument should be. Practice, practice, practice until your technique is second nature, and you can focus on the art of the music itself.
Here's a good example:
http://www.youtube.com/watch?v=GOTy3yhCylw
Notice how relaxed his right hand is, nee, his entire body, even on flourishes. Being relaxed also is more efficient and economical as far as using energy and lets you therefore play much faster, if the song calls for it.
Slapping with your thumb really just means twisting/rotating your hand, like the way you would turn your arm to go from looking at the inside of your wrist to looking at your watch. You should not be using the muscles in your hand or shoulder at all,
only your forearm muscles. Here's an exercise you can do to teach yourself to use the correct ones: Let your right arm hang loosely at your side, extend your thumb, and then tap yourself on the front of your leg using the side of your thumb by twisting/rotating your arm from the wrist down. Your forearm may rotate slightly in this process, but your upper arm (bicep/tricep area) should be totally still. This is how you slap, except that you will need to bend your elbow in order to bring it to the right height for playing your bass. It should be effortless and the weight of your hand, wrist, and distal (farthest from your shoulder) half of your forearm should be all the pressure you need on the strings.
Hope this helps.
