Ray I have always been amazed by players that can use their thumb as an extra "plucker" in slap. For some reason it always feels really awkward for me to even begin to use my thumb nail in the thumb up/thumb down technique. Then to add even more trouble to the mix I can't seem to get that pattern to flow with standard slap tehcnique. Do you have any particular exercises that you could suggest to really get the thumb combo with standard slap technique feeling more comfortable? How do you incorporate speed into this as well?

Using my thumb like a pick (double thumbing) was probably the hardest thing for me to get used to and get good at. My advice would be just a lot of practice. It's really a matter of just getting the "feel" of it. Make sure you don't dig to deep on the up stroke, just enough to let the string catch your thumb. Also remeber to stay relaxed and play light, there's no need to over do it.

What helped me was to play scales using double thumbing, try eigth and sixteenth note feels. As with anything new start slow and gradually increase speed as it get's more comfortable.

good luck! :)

I get what you are saying about playing light. It does seem easier to do this when I play light, but then of course I have the uneven sound especially on the up stroke. I know, I know...just keep practicing. As far as the scale practicing goes is it best to make it a habbit to always alternate thumb up thumb down or is making exceptions ok like when you change to the next string doing two thumb downs? I am thinking for long term where sometimes always doing alternate seems more beneficial (faster and evenly spaced).

Great question! I have a formula for scales that I love to use for the slap style of bass playing.
For technique I use this sequence: THUMB UP - THUMB DOWN - POP.
As for the scale I play two notes on the same string (thumb up- thumb down)followed by a pop on the next string, stay on that sting, two notes (thumb up-thumb down) followed by a pop on the next string...etc.

I have a lesson posted at THE INTERNATIONAL INSTITUTE OF BASS that explains this in detail:
http://www.instituteofbass.com/lessons/ray_riendeau/linear_slapping/