Slap bass is one of the best ways to bring out the basses sound, whether it be in funk (Red 
hot chili peppers bassist flea plays slap in most songs) all the way to alternative rock/emo 
(Coheed and cambria's bassist is also big for slap) You can hear a distinc sound of the 
bass when mastering slap.

There are two major ways to play slap, first you need to distinguish if your going to play 
it through the duriation of the song, or for only certain notes throughout the song. 
The quick slap method is to simply remove your thumb from your pick up, and slap, right 
above the bridge (Remember if you slap to close the the pick ups the strings will smack 
them and make a very clingy noise)by stiffening your thumb, and enclosing the rest of 
your hand into a fist and quickly hitting, than releasing off of the intended string 
and than to place your thumb back on the pick up, here's a quick exersise to try that.

S= Slap. Will be located either above or under the fret indicated to slap.

G—————10—12———————9—10——————10—12————————10—9—3
D——————————————————————————————————————————————
A——————————————————————————————————————————————
E——3—————————1—3—————————3——————————1—3————————
             S S                    S S
(Taken from,Red hot chili peppers, all around the world)


To play full slap you practically do the same thing, but most of the time its more vigurous 
and you have to maintain full follow through with your hand and the string. 
Lets give it a shot really quick

G——————————————
D——————————————
A————————55—7—5
E——33—77———————
   ss ss ss s s

If you had trouble with that, its normal. It will take a while to be able to maintain follow 
through, but I suggest if you really wanna learn how to play slap bass, play your scales 
using slap, play all the songs you know using slap. This lesson by Jeremy Honaker.