Fingerstyle, slap, and plectrum are the most common bass guitar techniques. There are loads of other though and perhaps the most remarkable is Bernard Edwards' 'chucking' technique. It involves using your fingers like a plectrum and it creates impossibly funky lines.
This video shows one of the best basslines to use the technique:
Here's a video lesson breaking the technique down a little more with a couple of exercises to work on.
This exercise works on coordinating the interplay between the hands.
This is a little trickier as you have to skip a string, so there's a little jump involved in the picking hand.
Check out Bernard in action here:
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