I wear clothespins. On a regular basis.
Mostly when I'm sitting in front of my computer working on a story. I have two wooden ones (I tested all the ones in the package to find the ones with the most tension).
The clothespin actual grip goes around the base of my nipple, which crushes the rest of the nipple back in the flat part of the pin. I have trained myself to these very slowly over the course of a year; I can wear them/bear the pain of them for an hour and a half, and then i can't control myself anyl onger, i HAVE to take them off. When i take them off, the tip of the nipple that was trapped in the flat part of the pin is all purple, and the rest of it is red. It takes a moment for the pain of them coming off to hit, but when it does its like red fireworks exploding in the blackness behind my eyelids. I have to fight the urge to cry out.
When I'm feeling a little more savage, I'll take the weights I have for them and put them on. I have five one ounce fishing weights for each clothespin. With those on, i can only take the pain for 45 minutes, when the throbbing becomes unbearable and i take them off. Again, with the same result.
I do have a set of those adjustable clamps, that I bought from a supply store on the 'Net, but i use them for my pussy lips, not my tits. the clothespins open wider (i have very fat nipples; they're the size of my pinky fingertip) and the adjustable clamps don't open that wide. They do provide more instantaneous pain, but if your sub has fat nipples, the clamps might not work. Clothespins do produce pain, but it can be a slow buildup, depending on where the subject's pain threshold is. When they come off, though...!
Don't try clamping clothespins to any part of your hand. The hand has more nerves per square inch of skin than any other part except the scrotal sac and perineal area on a guy; in a girl, the number of nerves is only matched in the clit, the tip of the breast, and in the perineal region (the skin between your sexual organs and your solid waste elimination area).
I hope this helps!