#1
Can somebody tell me some exercises for the slap technique people like john mayer use.? Its freakin Hard
#2
Practice.

Flick your index or middle finger while you make the percussive slap with the thumb on a bass string.

It's a really simple technique and when you get it down, you won't be able to understand why it was that hard to learn. There are plenty of Youtube videos on the technique as well.