Hey I was wondering if anybody could give me suggestions or advice on how to correctly slap the bass strings with your thumb. For example... Neon by John Mayer and Phunkdified by Justin King. I can't get a very loud note, usually just the clicking noise of the string hitting the fretboard and a faint note. Thanks guys.
Don't think of it as just a slap...its more of slap/pluck. Its hard to describe...but instead of an up down motion, think of it as pivot of your wrist. I pronate my hand to strike the bass note, and supinate my hand to pluck the the arpeggios, using my "wrist" as a pivot.

Partscaster/Tele into a bunch of pedals, a Maz 18 head, and a Z Best cab.