In addition to 'practice' per se, pain tolerance goes up with mental attention to the situation.

I'm sure you realize you can pinch yourself far harder and for far longer than you can tolerate from someone else. So practice creating the pain in yourself. Just knowing you can stop anytime makes the pain more tolerable. Once you have that going... you will also realize that some of the pain you feel from others really isn't as great as what you can tolerate from yourself. Then, suddenly, you will tolerate that pain.

Try pinching. Try clothespins and clover clamps. Get a hairbrush or pingpong paddle and use them on yourself.

Have a session where s/he give you a stroke and then hands the paddle to you and you give yourself two. Then s/he gives you one and you give yourself two more.

I'll bet you raise your play level real high.