The "Buddha rope" technique described by Bill 'The Buddha' Dickens in his training DVD basically entails putting a rope between strings and then using the increased tension for building muscles in your hands consistently. I've tried looking for a video to explain the technique, but Youtube seems to have removed most copies.
This technique done consistently (daily) works great for fingerstyle (speed and endurance).
Are there similar exercises (or consistency hacks) for getting your thumb technique, slapping and popping or even tapping techniques very consistent?