Td— Thumb Downwards 
Tu— Thumb Upwards
Pi— Pluck With Index Finger
Pm— Pluck With Middle Finger

This Is A Great Exercise That I Saw On You Tube And Its A Really 
Great Exercise For Slaps. 

The first thing you do is take your thumb and (Much Like A Pick) 
hit the note in a downwards motion and then take your thumb and 
hit it in a upwards motion which will lokk like this

G—————————————————————————————————|
D—————————————————————————————————|
A—5———5———5———5———5———5———5———5———|
E—————————————————————————————————|
  1   +   2   +   3   +   4   +
  Td  Tu  Td  Tu  Td  Tu  Td  Tu

Then work this with a Pluck with your index finger like such.

G—————————————————————————————————————|
D—————————————————————————————————————|
A—5——5——5——5——5——5——5——5——5——5——5——5——|
E—————————————————————————————————————|
  1  e  a  2  e  a  3  e  a  4  e  a
  Td Tu Pi Td Tu Pi Td Tu Pi Td Tu Pi

After that exercise throw in a pluck with your middle finger as well

G—————————————————————————————————————————————————————————————————|
D—————————————————————————————————————————————————————————————————|
A—5———5———5———5———5———5———5———5———5———5———5———5———5———5———5———5———|
E—————————————————————————————————————————————————————————————————|
  1   e   +   a   2   e   +   a   3   e   +   a   4   e   +   a
  Tb  Tu  Pi  Pm  Tb  Tu  Pi  Pm  Tb  Tu  Pi  Pm  Tb  Tu  Pi  Pm

If you work this with some scales it helps, but the main point 
of this exercise is to learn how to play fast with out having the 
need to use a pick.