Fieldy from Korn uses a slap tech that is a must learn for any bass enthusiast.
The technique involves using you pinkie finger and the backside of your hand to slap.

The way I mastered the technique was by slapping my thumb on a desk then following it with 
a slap from my pinkie sounds a bit weird but if you start doing it on the bass straight away 
it will probably not work.

When you slap with your pinkie try to keep it in line with the edge of your hand. I find the 
easiest way to do this is by keeping your ring finger pressed against your pinkie.

The next thing is building the strength of your rear hand slap. You need to make sure you 
are getting and almost equal amount of force out of the rear hand as you are with your thumb. 
This tends to be an extremely hard thing to do at first but as you keep trying your pinkie 
will become stronger and then it will be ready to tackle some serious slap songs.

Your hand must hang vertically along the strings when using the tech you cannot do it 
horizontally.

Some songs that Fieldy uses this in are:  Got the life (an extremely hard song to play), 
freak on a leash ( during chorus easier), right now (a hard slap song but tech only used 
in interlude takes some timing but fairly easy).

This tech is near impossible to use on anything but the top string

Some riffs you could try are.

Play fast 

(a hard one)
G:———————————————————————|
D:———————————————————————|
A:———————————————————————|
E:———2—0—XXX—0—XXX—2—3———|
           T T TTT T TTT T T
(an easier one) slower
G:———————————————————————|
D:———————————————————————|
A:———————————————————————|
E:———0—0—12—0—0—1—0—XXXX—|
           T T T  T T T T TTTT