I found a GREAT popping tutorial vid that we used to learn how to pop them. It does take a few tries, and you may have to refer back to it a few times, but it does the job!
YouTube - Ball Python Male
The important points: basically, you put your thumb RIGHT at the cloaca--not any further back, or it won't work.
Pull the tail back--don't worry, as long as you're reasonably gentle, no harm is done.
Roll the side of your thumb along the tail toward the cloaca. I found this to be much easier, and to work more reliably than some methods where the thumb is simply slid toward the snake's cloaca.
Snakes are fairly sturdy--you would have to press quite hard to injure them. It doesn't take a lot of force to pop a snake, it's more about using the correct technique.
Here are a few more good ones:
YouTube - How to Pop Ball Pythons
YouTube - PRO Poppin.........yeah booyeeeeeee
